x86/usercopy: Include arch_wb_cache_pmem() declaration

arch_wb_cache_pmem() is declared in a global header but defined by
the architecture. On x86, the implementation needs to include the header
to avoid this warning:

arch/x86/lib/usercopy_64.c:39:6: error: no previous prototype for 'arch_wb_cache_pmem' [-Werror=missing-prototypes]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Link: https://lore.kernel.org/all/20230516193549.544673-18-arnd%40kernel.org