include/cpu/x86/mp.h: Remove indirect include

This one might conflict with '#include <smp/atomic.h>'.

Change-Id: I7413406ca69e78e5a6e539a01e05033243107272
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/59691
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/include/cpu/x86/mp.h

@@ -3,7 +3,6 @@
 #ifndef _X86_MP_H_
 #define _X86_MP_H_
 
-#include <arch/smp/atomic.h>
 #include <cpu/x86/smm.h>
 #include <types.h>