x86/entry/common: Make prepare_exit_to_usermode() static

No users outside this file anymore.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lkml.kernel.org/r/20200708192934.301116609@linutronix.de

arch/x86/entry/common.c

@@ -294,7 +294,7 @@ static void __prepare_exit_to_usermode(struct pt_regs *regs)
 #endif
 }
 
-__visible noinstr void prepare_exit_to_usermode(struct pt_regs *regs)
+static noinstr void prepare_exit_to_usermode(struct pt_regs *regs)
 {
 	instrumentation_begin();
 	__prepare_exit_to_usermode(regs);