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/tftp.c b/net/tftp.c
index 34488b7..5a69bca 100644
--- a/net/tftp.c
+++ b/net/tftp.c
@@ -9,6 +9,7 @@
 #include <common.h>
 #include <command.h>
 #include <efi_loader.h>
+#include <env.h>
 #include <mapmem.h>
 #include <net.h>
 #include <net/tftp.h>