uboot-envtools: fix build with musl

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

SVN-Revision: 39961

package/boot/uboot-envtools/patches/100-fw_env_gnu_source.patch

+--- a/tools/env/fw_env.c
++++ b/tools/env/fw_env.c
+@@ -8,6 +8,7 @@
+  * SPDX-License-Identifier:	GPL-2.0+
+  */
+ 
++#define _GNU_SOURCE
+ #include <errno.h>
+ #include <env_flags.h>
+ #include <fcntl.h>