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/cmd/dtimg.c b/cmd/dtimg.c
index ae7d82f..6c5d53c 100644
--- a/cmd/dtimg.c
+++ b/cmd/dtimg.c
@@ -4,6 +4,7 @@
  * Sam Protsenko <semen.protsenko@linaro.org>
  */
 
+#include <env.h>
 #include <image-android-dt.h>
 #include <common.h>