common: Drop linux/stringify.h from common header

Move this uncommon header out of the common header.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/configs/pico-imx6ul.h b/include/configs/pico-imx6ul.h
index b0e2f6c..8bfada7 100644
--- a/include/configs/pico-imx6ul.h
+++ b/include/configs/pico-imx6ul.h
@@ -111,6 +111,7 @@
 	func(DHCP, dhcp, na)
 
 #include <config_distro_bootcmd.h>
+#include <linux/stringify.h>
 
 #define CONFIG_SYS_LOAD_ADDR		CONFIG_LOADADDR
 #define CONFIG_SYS_HZ			1000