chore: meson: Remove posix dep

[ryonakano]

This seems to be originally added to the meson.build in ceea22f by me, I didn't remember though, but I don't feel it's not used in the source code at all then and now. Maybe I brought it to the meson.build since it was in the CMake files? Anyways I think we can safely remove it from the dependencies.

user@elementary-daily:~/work/pantheon-tweaks ((ceea22f...))$ git grepn -i posix
cmake/README.Vala.rst:121:        posix
cmake/ValaPrecompile.cmake:96:#       posix
meson.build:64:        meson.get_compiler('vala').find_library('posix'),
src/CMakeLists.txt:52:    posix
user@elementary-daily:~/work/pantheon-tweaks ((ceea22f...))$