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>

common/spl/spl_fat.c

@@ -9,6 +9,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include <spl.h>
 #include <asm/u-boot.h>
 #include <fat.h>