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>

env/callback.c

@@ -5,6 +5,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include <environment.h>
 
 #if defined(CONFIG_NEEDS_MANUAL_RELOC)