compositor: add missing help text

The headless-backend.so was missing in available backend list

Signed-off-by: JoonCheol Park <jooncheol@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

src/compositor.c

@@ -5043,6 +5043,9 @@ usage(int error_code)
 #if defined(BUILD_RDP_COMPOSITOR)
 			"\t\t\t\trdp-backend.so\n"
 #endif
+#if defined(BUILD_HEADLESS_COMPOSITOR)
+			"\t\t\t\theadless-backend.so\n"
+#endif
 #if defined(BUILD_RPI_COMPOSITOR) && defined(HAVE_BCM_HOST)
 			"\t\t\t\trpi-backend.so\n"
 #endif