[vcpkg] Fix the order of structures to adapt to the linu kernel

[FrankXie05]

Hi, all. I am the maintainer of vcpkg.

Recently we received a build error regarding gloo. microsoft/vcpkg#38852

Reproduce:

git clone https://github.com/microsoft/vcpkg
cd vcpkg/
./vcpkg install gloo:x64-linux

Error:

FAILED: gloo/CMakeFiles/gloo.dir/common/linux.cc.o 
/usr/bin/c++  -I/mnt/c/Kiko/Tools/vcpkg/buildtrees/gloo/src/4842d821e2-a23aea63a5.clean -I/mnt/c/Kiko/Tools/vcpkg/buildtrees/gloo/x64-linux-dbg -fPIC -std=c++11 -fPIC -g -MD -MT gloo/CMakeFiles/gloo.dir/common/linux.cc.o -MF gloo/CMakeFiles/gloo.dir/common/linux.cc.o.d -o gloo/CMakeFiles/gloo.dir/common/linux.cc.o -c /mnt/c/Kiko/Tools/vcpkg/buildtrees/gloo/src/4842d821e2-a23aea63a5.clean/gloo/common/linux.cc
In file included from /mnt/c/Kiko/Tools/vcpkg/buildtrees/gloo/src/4842d821e2-a23aea63a5.clean/gloo/common/linux.cc:15:
/usr/include/linux/ethtool.h: In function ‘int gloo::getInterfaceSpeedGLinkSettings(int, ifreq*)’:
/usr/include/linux/ethtool.h:2214:17: error: flexible array member ‘ethtool_link_settings::link_mode_masks’ not at end of ‘struct gloo::getInterfaceSpeedGLinkSettings(int, ifreq*)::<unnamed>’
 2214 |         __u32   link_mode_masks[];
      |                 ^~~~~~~~~~~~~~~
/mnt/c/Kiko/Tools/vcpkg/buildtrees/gloo/src/4842d821e2-a23aea63a5.clean/gloo/common/linux.cc:192:11: note: next member ‘__u32 gloo::getInterfaceSpeedGLinkSettings(int, ifreq*)::<unnamed struct>::link_mode_data [381]’ declared here
  192 |     __u32 link_mode_data[link_mode_data_nwords];
      |           ^~~~~~~~~~~~~~
/mnt/c/Kiko/Tools/vcpkg/buildtrees/gloo/src/4842d821e2-a23aea63a5.clean/gloo/common/linux.cc:190:10: note: in the definition of ‘struct gloo::getInterfaceSpeedGLinkSettings(int, ifreq*)::<unnamed>’
  190 |   struct {
      |          ^

Solution:
I think the flexible array member link_mode_masks should be at the end of the structure, otherwise it is illegal in C++.

And this structure requires dynamic allocation of memory to ensure that the flexible array members have enough space to store the required data.

I reorganized the structure and used malloc for dynamic allocation of memory.
I will get your help as soon as possible. :)

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[FrankXie05]

In order to merge any pull request (code, docs, etc.), we require contributors to sign our Contributor License Agreement, and we don't seem to have one on file for you.

Done.

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[FrankXie05]

@malfet

[dg0yt]

This is fairly invasive PR, changing structs and adding dynamic memory allocations.
And the actual changes blured by changes of indent. Can you please change the indents to the original ones?

My first impression is that the original cause is a compiler bug.
And there might be much less invasive workarounds.
But this is hard to comment with the current formatting.

[dg0yt]

AFAIU this is not a "flexible array". link_mode_data_nwords is constexpr, so it is very much the same as a literal 381.