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/drivers/net/sh_eth.c b/drivers/net/sh_eth.c
index 2d5c970..e2ca474 100644
--- a/drivers/net/sh_eth.c
+++ b/drivers/net/sh_eth.c
@@ -10,6 +10,7 @@
 
 #include <config.h>
 #include <common.h>
+#include <env.h>
 #include <environment.h>
 #include <malloc.h>
 #include <net.h>