x86: Move gds_ucode_mitigated() declaration to header

The declaration got placed in the .c file of the caller, but that
causes a warning for the definition:

arch/x86/kernel/cpu/bugs.c:682:6: error: no previous prototype for 'gds_ucode_mitigated' [-Werror=missing-prototypes]

Move it to a header where both sides can observe it instead.

Fixes: 81ac7e5 ("KVM: Add GDS_NO support to KVM")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Tested-by: Daniel Sneddon <daniel.sneddon@linux.intel.com>
Cc: stable@kernel.org
Link: https://lore.kernel.org/all/20230809130530.1913368-2-arnd%40kernel.org

arch/x86/include/asm/processor.h

@@ -731,4 +731,6 @@ bool arch_is_platform_page(u64 paddr);
 #define arch_is_platform_page arch_is_platform_page
 #endif
 
+extern bool gds_ucode_mitigated(void);
+
 #endif /* _ASM_X86_PROCESSOR_H */

arch/x86/kvm/x86.c

@@ -314,8 +314,6 @@ u64 __read_mostly host_xcr0;
 
 static struct kmem_cache *x86_emulator_cache;
 
-extern bool gds_ucode_mitigated(void);
-
 /*
  * When called, it means the previous get/set msr reached an invalid msr.
  * Return true if we want to ignore/silent this failed msr access.