env: Move env_set() to env.h

Move env_set() over to the new header file.

Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/image-android.c b/common/image-android.c
index 6c9568a..264bf90 100644
--- a/common/image-android.c
+++ b/common/image-android.c
@@ -4,6 +4,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include <image.h>
 #include <android_image.h>
 #include <malloc.h>