sparc64: fix building assembly files

linux/posix_types.h must not be included in assembler files,
so move the #include statement down into the appropriate
ifdef section.

Fixes: 72113d0a ("signal.h: add linux/signal.h and asm/signal.h to UAPI compile-test coverage")
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/linux-arch/202202172154.lJ3Z0yXe-lkp@intel.com/


Signed-off-by: Arnd Bergmann <arnd@arndb.de>