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/fastboot/fb_mmc.c b/drivers/fastboot/fb_mmc.c
index 0a335db..b0b19c5 100644
--- a/drivers/fastboot/fb_mmc.c
+++ b/drivers/fastboot/fb_mmc.c
@@ -6,6 +6,7 @@
 #include <config.h>
 #include <common.h>
 #include <blk.h>
+#include <env.h>
 #include <fastboot.h>
 #include <fastboot-internal.h>
 #include <fb_mmc.h>