env: Move env_get_hex() to env.h

Move env_get_hex() over to the new header file.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>

common/image-fdt.c

@@ -11,6 +11,7 @@
 #include <common.h>
 #include <fdt_support.h>
 #include <fdtdec.h>
+#include <env.h>
 #include <errno.h>
 #include <image.h>
 #include <linux/libfdt.h>