x86/kprobes: Remove trampoline_handler() prototype

... and make it static. It is called only by the kretprobe_trampoline()
from asm.

It was marked __visible so that it is visible outside of the current
compilation unit but that is not needed as it is used only in this
compilation unit.
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Link: https://lkml.kernel.org/r/20181205162526.GB109259@gmail.com

arch/x86/kernel/kprobes/core.c

@@ -66,8 +66,6 @@
 
 #include "common.h"
 
-void *trampoline_handler(struct pt_regs *regs);
-
 DEFINE_PER_CPU(struct kprobe *, current_kprobe) = NULL;
 DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);
 
@@ -753,7 +751,7 @@ STACK_FRAME_NON_STANDARD(kretprobe_trampoline);
 /*
  * Called from kretprobe_trampoline
  */
-__visible __used void *trampoline_handler(struct pt_regs *regs)
+static __used void *trampoline_handler(struct pt_regs *regs)
 {
 	struct kretprobe_instance *ri = NULL;
 	struct hlist_head *head, empty_rp;