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/net/link_local.c b/net/link_local.c
index 31cdef4..dd9fcad 100644
--- a/net/link_local.c
+++ b/net/link_local.c
@@ -12,6 +12,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include <net.h>
 #include "arp.h"
 #include "net_rand.h"