powerpc/ptrace: Add prototype for function pt_regs_check

`pt_regs_check` is a dummy function, its purpose is to break the build
if struct pt_regs and struct user_pt_regs don't match.

This function has no functionnal purpose, and will get eliminated at
link time or after init depending on CONFIG_LD_DEAD_CODE_DATA_ELIMINATION

This commit adds a prototype to fix warning at W=1:

  arch/powerpc/kernel/ptrace.c:3339:13: error: no previous prototype for ‘pt_regs_check’ [-Werror=missing-prototypes]
Suggested-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20181208154624.6504-1-malat@debian.org

arch/powerpc/kernel/ptrace.c

@@ -3361,6 +3361,12 @@ void do_syscall_trace_leave(struct pt_regs *regs)
 	user_enter();
 }
 
+void __init pt_regs_check(void);
+
+/*
+ * Dummy function, its purpose is to break the build if struct pt_regs and
+ * struct user_pt_regs don't match.
+ */
 void __init pt_regs_check(void)
 {
 	BUILD_BUG_ON(offsetof(struct pt_regs, gpr) !=