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>

drivers/net/fm/fdt.c

@@ -3,6 +3,7 @@
  * Copyright 2016 Freescale Semiconductor, Inc.
  */
 #include <asm/io.h>
+#include <env.h>
 #include <fsl_qe.h>	/* For struct qe_firmware */
 
 #ifdef CONFIG_SYS_DPAA_FMAN