arm64: move cpu_suspend_set_dbg_restorer() prototype to header

The cpu_suspend_set_dbg_restorer() function is called by the hw_breakpoint
code but defined in another file. Since the declaration is in the
same file as the caller, the compiler warns about the definition
without a prior prototype:

arch/arm64/kernel/suspend.c:35:13: error: no previous prototype for 'cpu_suspend_set_dbg_restorer' [-Werror=missing-prototypes]

Move it into the corresponding header instead to avoid the warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20230516160642.523862-5-arnd@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>