include/cpu/x86/pae.h: Add missing include

Add the include for size_t.
Fixes compilation error on source files that do not include it.

Change-Id: Ic752886d94db18de89b8b8a5e70cf03965aeb5c3
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/31922
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/include/cpu/x86/pae.h

@@ -2,6 +2,7 @@
 #define CPU_X86_PAE_H
 
 #include <stdint.h>
+#include <stddef.h>
 
 /* Enable paging with cr3 value for page directory pointer table as well as PAE
    option in cr4. */