hw-breakpoints: Fix build warnings with clang

hw-breakpoints: Fix build warnings with clang

kbuild test robot reported some build warnings in the hw_breakpoint
code when compiled with clang[1]. Some of them were introduced by the
recent powerpc change to add arch_reserve_bp_slot() and
arch_release_bp_slot(). Fix them all.

  kernel/events/hw_breakpoint.c:71:12: warning: no previous prototype for function 'hw_breakpoint_weight'
  kernel/events/hw_breakpoint.c:216:12: warning: no previous prototype for function 'arch_reserve_bp_slot'
  kernel/events/hw_breakpoint.c:221:13: warning: no previous prototype for function 'arch_release_bp_slot'
  kernel/events/hw_breakpoint.c:228:13: warning: no previous prototype for function 'arch_unregister_hw_breakpoint'

[1]: https://lore.kernel.org/linuxppc-dev/202005192233.oi9CjRtA%25lkp@intel.com/

Fixes: 29da4f91c0c1 ("powerpc/watchpoint: Don't allow concurrent perf and ptrace events")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
[mpe: Drop extern, flesh out change log, add Fixes tag]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200602041208.128913-1-ravi.bangoria@linux.ibm.com