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>

common/usb_hub.c

@@ -24,6 +24,7 @@
 #include <common.h>
 #include <command.h>
 #include <dm.h>
+#include <env.h>
 #include <errno.h>
 #include <memalign.h>
 #include <asm/processor.h>