tests: use statx instead of sendmmsg in has_function test

Both recvmmsg/sendmmsg and statx require the `_GNU_SOURCE` symbol to be
defined to be shown in headers, but statx has the additional property of
being defined on all glibc systems[1], unlike recvmmsg which isn't
implemented on Hurd[2][3].

With this fix, all tests now pass on Hurd.

    $ cat meson.build
    project('test', 'c')
    cc = meson.get_compiler('c')
    cc.has_header_symbol('sys/stat.h', 'statx')
    cc.has_header_symbol('sys/stat.h', 'statx', prefix: '#define _GNU_SOURCE')
    cc.has_function('statx')

    $ meson setup build
    The Meson build system
    Version: 1.5.1
    Source dir: /tmp/tmp.XcWk4fMra4
    Build dir: /tmp/tmp.XcWk4fMra4/build
    Build type: native build
    Project name: test
    Project version: undefined
    C compiler for the host machine: cc (gcc 14.2.0 "cc (Debian 14.2.0-3) 14.2.0")
    C linker for the host machine: cc ld.bfd 2.43.1
    Host machine cpu family: x86_64
    Host machine cpu: x86_64
    Header "sys/stat.h" has symbol "statx" : NO
    Header "sys/stat.h" has symbol "statx" : YES
    Checking for function "statx" : YES
    Build targets in project: 0

[1]: https://sourceware.org/git/?p=glibc.git;a=blob;f=io/statx_generic.c;h=93faf12297cfcd37ba005a02e991eab351d74174;hb=HEAD
[2]: https://www.gnu.org/software/hurd/open_issues/glibc.html#recvmmsg
[3]: https://github.com/mesonbuild/meson/pull/4351

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