tools/env: Fix build failure from missing header include

This was introduced in:
8679d0ffdcc0beafea8e6942c0c67cf859afa18e -
	COMMON: Use __stringify() instead of MK_STR()

The header is now needed since common.h is not included in this tool.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>

tools/env/fw_env.c

@@ -26,6 +26,7 @@
 
 #include <errno.h>
 #include <fcntl.h>
+#include <linux/stringify.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <stddef.h>