uboot-envtools: fix build with musl

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39961 3c298f89-4303-0410-b956-a3cf2f4a3e73

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

@@ -0,0 +1,10 @@
+--- 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>