x86/irqflags: Mark native_restore_fl extern inline

This should have been marked extern inline in order to pick up the out
of line definition in arch/x86/kernel/irqflags.S.

Fixes: 208cbb3 ("x86/irqflags: Provide a declaration for native_save_fl")
Reported-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20180827214011.55428-1-ndesaulniers@google.com

Author: nickdesaulniers

arch/x86/include/asm/irqflags.h

@@ -33,7 +33,8 @@ extern inline unsigned long native_save_fl(void)
 	return flags;
 }
 
-static inline void native_restore_fl(unsigned long flags)
+extern inline void native_restore_fl(unsigned long flags);
+extern inline void native_restore_fl(unsigned long flags)
 {
 	asm volatile("push %0 ; popf"
 		     : /* no output */