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/nvram.c b/env/nvram.c
index df1b379..a5b1873 100644
--- a/env/nvram.c
+++ b/env/nvram.c
@@ -25,6 +25,7 @@
 
 #include <common.h>
 #include <command.h>
+#include <env.h>
 #include <environment.h>
 #include <linux/stddef.h>
 #include <search.h>