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>

drivers/net/ftmac100.c

@@ -8,6 +8,7 @@
 
 #include <config.h>
 #include <common.h>
+#include <env.h>
 #include <malloc.h>
 #include <net.h>
 #include <linux/io.h>