meson: Test to build xserver_poll.c was inverted

Test to build xserver_poll.c was inverted compared to autoconf. Build
xserver_poll.c if poll is missing.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>

os/meson.build

@@ -42,10 +42,10 @@ endif
 if not cc.has_function('timingsafe_memcmp')
     srcs_libc += 'timingsafe_memcmp.c'
 endif
-
-if cc.has_function('poll')
+if not cc.has_function('poll')
     srcs_os += 'xserver_poll.c'
 endif
+
 if cc.has_function('sigaction')
     srcs_os += 'busfault.c'
 endif