xtensa: only build __strncpy_user with CONFIG_ARCH_HAS_STRNCPY_FROM_USER

__strncpy_user is only used when CONFIG_ARCH_HAS_STRNCPY_FROM_USER is
enabled so don't build the source when it's off.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/lib/Makefile

@@ -6,7 +6,8 @@
 lib-y	+= memcopy.o memset.o checksum.o \
 	   ashldi3.o ashrdi3.o bswapdi2.o bswapsi2.o lshrdi3.o \
 	   divsi3.o udivsi3.o modsi3.o umodsi3.o mulsi3.o umulsidi3.o \
-	   usercopy.o strncpy_user.o strnlen_user.o
+	   usercopy.o strnlen_user.o
+lib-$(CONFIG_ARCH_HAS_STRNCPY_FROM_USER) += strncpy_user.o
 lib-$(CONFIG_PCI) += pci-auto.o
 lib-$(CONFIG_KCSAN) += kcsan-stubs.o
 KCSAN_SANITIZE_kcsan-stubs.o := n