fix FTBS on missing asprintf() prototype on GNU/Linux

On GNU/Linux we have to define _GNU_SOURCE before including stdio.h,
in order to get the prototype for asprintf().

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

src/qxl_driver.c

@@ -32,6 +32,10 @@
 #include "config.h"
 #endif
 
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+
 #include <unistd.h>
 #include <string.h>
 #include <stdio.h>

src/qxl_kms.c

@@ -24,6 +24,12 @@
 #include <config.h>
 #endif
 
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+
+#include <stdio.h>
+
 #ifdef XF86DRM_MODE
 #include <fcntl.h>
 #include <sys/mman.h>