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_common.c b/drivers/fastboot/fb_common.c
index 17eca73..e76af8e 100644
--- a/drivers/fastboot/fb_common.c
+++ b/drivers/fastboot/fb_common.c
@@ -11,6 +11,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include <fastboot.h>
 #include <net/fastboot.h>