compat: Declare compat_sys_sigpending and compat_sys_sigprocmask prototypes

__ARCH_WANT_SYS_SIGPENDING or __ARCH_WANT_SYS_SIGPROGMASK may be defined
for compat support but the corresponding prototypes are missing from
linux/compat.h.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>