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/ftmac100.c b/drivers/net/ftmac100.c
index c08889c..d8f1dde 100644
--- a/drivers/net/ftmac100.c
+++ b/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>