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/board/renesas/silk/silk.c b/board/renesas/silk/silk.c
index 59ba262..bda6a41 100644
--- a/board/renesas/silk/silk.c
+++ b/board/renesas/silk/silk.c
@@ -7,6 +7,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include <malloc.h>
 #include <dm.h>
 #include <dm/platform_data/serial_sh.h>