fix lttng-tools fails to compile with libxml2 2.14.0+

[hongxu-jia]

Description:
| In file included from /srv/pokybuild/yocto-worker/qemux86-alt/build/build/tmp/work/core2-32-poky-linux/lttng-tools/2.13.15/recipe-sysroot/usr/include/libxml2/libxml/parser.h:25,
| from ../../../../lttng-tools-2.13.15/src/common/config/session-config.c:29:
| /srv/pokybuild/yocto-worker/qemux86-alt/build/build/tmp/work/core2-32-poky-linux/lttng-tools/2.13.15/recipe-sysroot/usr/include/libxml2/libxml/encoding.h:173:7: note: declared here
| 173 | } input XML_DEPRECATED_MEMBER;
| | ^~~~~
| ../../../../lttng-tools-2.13.15/src/common/config/session-config.c:432:15: error: called object is not a function or function pointer
| 432 | ret = handler->input(out_str, &out_len, (const xmlChar *) in_str, &in_len);
| | ^~~~~~~
| At top level:
| cc1: note: unrecognized command-line option '-Wno-incomplete-setjmp-declaration' may have been intended to silence earlier diagnostics

According to [1][2], the UTF-8 handler is

static xmlCharEncError
UTF8ToUTF8(void *vctxt ATTRIBUTE_UNUSED,
           unsigned char* out, int *outlen,
           const unsigned char* in, int *inlen,
           int flush ATTRIBUTE_UNUSED)

Update input.func with setting ATTRIBUTE_UNUSED params with NULL and 0

[1] https://gitlab.gnome.org/GNOME/libxml2/-/commit/38f475072aefe032fff1dc058df3e56c1e7062fa
[2] https://gitlab.gnome.org/GNOME/libxml2/-/commit/69b83bb68e2a8ed0013f80c51b9a358714b00c9a#478024cc18a2cc8dbaed34076e9775f6827f413d_2188_2201

[hongxu-jia]

From https://bugs.lttng.org/issues/1427, the issue has been fixed in lttng-tools master 1eadd11 but has not yet to be backported to stable-2.13.

Because the source code has been reworked from C to C++, it is hardly to backport directly, so this PR try to fix stable-2.13 with minor change