shared: include missing header to weston-assert

We forgot to include stdbool to weston-assert.h, but this was not
causing issues because callers of weston_assert_true() themselves
include stdbool.

Add stdbool to weston-assert, as this is the right thing to do.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>

shared/weston-assert.h

@@ -29,6 +29,7 @@
 #include <stdlib.h>
 #include <stdarg.h>
 #include <string.h>
+#include <stdbool.h>
 
 struct weston_compositor;