compositor/shared: Suppress write(2) warnings

Fixes the following warnings when building with _FORTIFY_SOURCE
and optimizations enabled:

../shared/xalloc.h:49:9: error: ignoring return value of ‘write’ declared with attribute ‘warn_unused_result’ [-Werror=unused-result]
   49 |         write(STDERR_FILENO, oommsg, strlen(oommsg));
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

or
../compositor/main.c:427:25: error: ignoring return value of ‘write’ declared with attribute ‘warn_unused_result’ [-Werror=unused-result]
  427 |                         write(STDERR_FILENO, fail_seteuid,
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  428 |                               strlen(fail_seteuid));
      |                               ~~~~~~~~~~~~~~~~~~~~~
../compositor/main.c:434:25: error: ignoring return value of ‘write’ declared with attribute ‘warn_unused_result’ [-Werror=unused-result]
  434 |                         write(STDERR_FILENO, fail_cloexec,
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  435 |                               strlen(fail_cloexec));
      |                               ~~~~~~~~~~~~~~~~~~~~~
../compositor/main.c:442:25: error: ignoring return value of ‘write’ declared with attribute ‘warn_unused_result’ [-Werror=unused-result]
  442 |                         write(STDERR_FILENO, fail_exec, strlen(fail_exec));
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>