xtensa: hw_breakpoint: include header for missing prototype

Add the prototype for restore_dbreak() to <asm/hw_breakpoint.h> and use
that header in hw_breakpoint.c to prevent a build warning:

arch/xtensa/kernel/hw_breakpoint.c:263:6: warning: no previous prototype for 'restore_dbreak' [-Wmissing-prototypes]
  263 | void restore_dbreak(void)

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Message-Id: <20230920052139.10570-12-rdunlap@infradead.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>