x86/cpu: Export native_write_cr4() only when CONFIG_LKTDM=m

Modules have no business poking into this but fixing this is for later.

 [ bp: Carve out from an earlier patch. ]
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200421092558.939985695@linutronix.de

arch/x86/kernel/cpu/common.c

@@ -387,7 +387,9 @@ void native_write_cr4(unsigned long val)
 			  bits_missing);
 	}
 }
+#if IS_MODULE(CONFIG_LKDTM)
 EXPORT_SYMBOL_GPL(native_write_cr4);
+#endif
 
 void cr4_update_irqsoff(unsigned long set, unsigned long clear)
 {