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/common/bootretry.c b/common/bootretry.c
index 072055b..47aaaa8 100644
--- a/common/bootretry.c
+++ b/common/bootretry.c
@@ -7,6 +7,7 @@
 #include <common.h>
 #include <bootretry.h>
 #include <cli.h>
+#include <env.h>
 #include <errno.h>
 #include <watchdog.h>