powerpc: Add missing prototypes for hw_breakpoint_handler() & arch_unregister_hw_breakpoint()

In commit 5aae8a53 ("powerpc, hw_breakpoints: Implement
hw_breakpoints for 64-bit server processors") function
hw_breakpoint_handler() and arch_unregister_hw_breakpoint() were added
without function prototypes in hw_breakpoint.h header.

Fix the following warning(s) (treated as error in W=1):
  arch/powerpc/kernel/hw_breakpoint.c:106:6: error: no previous prototype for ‘arch_unregister_hw_breakpoint’
  arch/powerpc/kernel/hw_breakpoint.c:209:5: error: no previous prototype for ‘hw_breakpoint_handler’
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/hw_breakpoint.h

@@ -66,6 +66,7 @@ extern int hw_breakpoint_exceptions_notify(struct notifier_block *unused,
 						unsigned long val, void *data);
 int arch_install_hw_breakpoint(struct perf_event *bp);
 void arch_uninstall_hw_breakpoint(struct perf_event *bp);
+void arch_unregister_hw_breakpoint(struct perf_event *bp);
 void hw_breakpoint_pmu_read(struct perf_event *bp);
 extern void flush_ptrace_hw_breakpoint(struct task_struct *tsk);
 
@@ -82,6 +83,7 @@ static inline void hw_breakpoint_disable(void)
 	__set_breakpoint(&brk);
 }
 extern void thread_change_pc(struct task_struct *tsk, struct pt_regs *regs);
+int hw_breakpoint_handler(struct die_args *args);
 
 #else	/* CONFIG_HAVE_HW_BREAKPOINT */
 static inline void hw_breakpoint_disable(void) { }