stackleak: add declarations for global functions

stackleak: add declarations for global functions

With -Wmissing-prototypes enabled, the stackleak code produces a couple of
warnings that have no declarations because they are only called from assembler:

stackleak.c:127:25: error: no previous prototype for 'stackleak_erase' [-Werror=missing-prototypes]
stackleak.c:139:25: error: no previous prototype for 'stackleak_erase_on_task_stack' [-Werror=missing-prototypes]
stackleak.c:151:25: error: no previous prototype for 'stackleak_erase_off_task_stack' [-Werror=missing-prototypes]
stackleak.c:159:49: error: no previous prototype for 'stackleak_track_stack' [-Werror=missing-prototypes]

Add declarations to the stackleak header to shut up the warnings.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20231108125843.3806765-7-arnd@kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>