meson: unbreak sysctl.h detection on BSDs

meson: unbreak sysctl.h detection on BSDs

Code:
 #include <sys/sysctl.h>
Compiler stdout:

Compiler stderr:
 In file included from testfile.c:1:
/usr/include/sys/sysctl.h:1184:40: error: unknown type name 'size_t'
int     sysctl(const int *, u_int, void *, size_t *, const void *, size_t);
                                           ^
/usr/include/sys/sysctl.h:1185:40: error: unknown type name 'size_t'
int     sysctlbyname(const char *, void *, size_t *, const void *, size_t);
                                           ^
/usr/include/sys/sysctl.h:1186:42: error: unknown type name 'size_t'
int     sysctlnametomib(const char *, int *, size_t *);
                                             ^
3 errors generated.

Checking if "sys/sysctl.h" compiles: NO

<https://gitlab.freedesktop.org/mesa/drm/-/commit/1f8ada802391>
<https://gitlab.freedesktop.org/mesa/drm/-/commit/4083e8f2c659>

Reviewed-by: Niclas Zeising <zeising@daemonic.se>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5462>