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/cmd/itest.c b/cmd/itest.c
index fd6f416..8b630d7 100644
--- a/cmd/itest.c
+++ b/cmd/itest.c
@@ -14,6 +14,7 @@
 #include <common.h>
 #include <config.h>
 #include <command.h>
+#include <env.h>
 #include <mapmem.h>
 
 #include <asm/io.h>