x86: Add a forward struct declaration in coreboot_tables.h

This struct is not defined in this header file. Add a forward declaration
so that it can be included in any context.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

arch/x86/include/asm/coreboot_tables.h

@@ -8,6 +8,8 @@
 #ifndef _COREBOOT_TABLES_H
 #define _COREBOOT_TABLES_H
 
+struct memory_area;
+
 struct cbuint64 {
 	u32 lo;
 	u32 hi;