mtd: fix compile error with musl

Signed-off-by: Felix Fietkau <[email protected]>

SVN-Revision: 40819

package/system/mtd/src/mtd.c

@@ -21,6 +21,7 @@
  * The code is based on the linux-mtd examples.
  */
 
+#define _GNU_SOURCE
 #include <limits.h>
 #include <unistd.h>
 #include <stdlib.h>