build: Enable C99 to fix old systems

In Epel7 the default is not C99 but we are using some C99
syntax. Enable C99 to avoid compile errors.
Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Acked-by: Uri Lublin <uril@redhat.com>

meson.build

@@ -9,6 +9,7 @@ project('spice-streaming-agent', 'c', 'cpp',
           'buildtype=debugoptimized',
           'warning_level=2',
           'cpp_std=gnu++11',
+          'c_std=gnu99',
         ])
 
 meson.add_dist_script('build-aux/meson-dist', meson.source_root())