drivers/intel/fsp2_0: Include stddef.h in soc_binding.h

soc_binding.h includes FSP headers which define NULL macro. Because of
this, including stddef.h after soc_binding.h results in NULL being
re-defined. Thus, include stddef.h in soc_binding.h to avoid having
users include stddef.h along with soc_binding.h.

Change-Id: I600083c5d8f672518beaa1119f14f67728a433aa
Signed-off-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: https://review.coreboot.org/17773
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/drivers/intel/fsp2_0/include/fsp/soc_binding.h

@@ -16,6 +16,8 @@
 #ifndef _FSP2_0_SOC_BINDING_H_
 #define _FSP2_0_SOC_BINDING_H_
 
+#include <stddef.h>
+
 #pragma pack(push)
 /*
  * This file is a implementation specific header. i.e. different