arm64: move early_brk64 prototype to header

arm64: move early_brk64 prototype to header

The prototype used for calling early_brk64() is in the file that calls
it, which is the wrong place, as it is not included for the definition:

arch/arm64/kernel/traps.c:1100:12: error: no previous prototype for 'early_brk64' [-Werror=missing-prototypes]

Move it to an appropriate header instead.

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/20230516160642.523862-15-arnd@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>