dosfstools: fix build with musl

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

utils/dosfstools/Makefile

@@ -71,7 +71,7 @@ endef
 define Build/Compile
 	$(MAKE) -C $(PKG_BUILD_DIR) \
 		$(TARGET_CONFIGURE_OPTS) \
-		OPTFLAGS="$(TARGET_CFLAGS) -D_FILE_OFFSET_BITS=64" \
+		OPTFLAGS="$(TARGET_CFLAGS) -D_FILE_OFFSET_BITS=64 -D_GNU_SOURCE" \
 		PREFIX="/usr" \
 		SBINDIR="/usr/sbin" \
 		LDFLAGS="$(LDFLAGS)" \