uml: fix WARNING: vmlinux: 'memcpy' exported twice

Fix the following warning on x86_64:

LD vmlinux.o
MODPOST vmlinux.o
WARNING: vmlinux: 'memcpy' exported twice. Previous export was in vmlinux

For x86_64, this symbol is already exported from arch/um/sys-x86_64/ksyms.c.
Reported-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>
Tested-by: Boaz Harrosh <bharrosh@panasas.com>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/os-Linux/user_syms.c

@@ -14,7 +14,6 @@
 #undef memset
 
 extern size_t strlen(const char *);
-extern void *memcpy(void *, const void *, size_t);
 extern void *memmove(void *, const void *, size_t);
 extern void *memset(void *, int, size_t);
 extern int printf(const char *, ...);
@@ -24,7 +23,11 @@ extern int printf(const char *, ...);
 EXPORT_SYMBOL(strstr);
 #endif
 
+#ifndef __x86_64__
+extern void *memcpy(void *, const void *, size_t);
 EXPORT_SYMBOL(memcpy);
+#endif
+
 EXPORT_SYMBOL(memmove);
 EXPORT_SYMBOL(memset);
 EXPORT_SYMBOL(printf);