compiler.h: replace QEMU_WARN_UNUSED_RESULT with G_GNUC_WARN_UNUSED_RESULT

One less qemu-specific macro. It also helps to make some headers/units
only depend on glib, and thus moved in standalone projects eventually.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

include/qemu-common.h

@@ -27,7 +27,7 @@ int qemu_main(int argc, char **argv, char **envp);
 #endif
 
 ssize_t qemu_write_full(int fd, const void *buf, size_t count)
-    QEMU_WARN_UNUSED_RESULT;
+    G_GNUC_WARN_UNUSED_RESULT;
 
 #ifndef _WIN32
 int qemu_pipe(int pipefd[2]);