env: Move env_get() to env.h

Move env_get() 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/env/callback.c b/env/callback.c
index 54d2de4..95be80d 100644
--- a/env/callback.c
+++ b/env/callback.c
@@ -5,6 +5,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include <environment.h>
 
 #if defined(CONFIG_NEEDS_MANUAL_RELOC)