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 <[e-mail]>
Signed-off-by: WANG Cong <[e-mail]>
Tested-by: Boaz Harrosh <[e-mail]>
Cc: Jeff Dike <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/um/os-Linux/user_syms.c

···
 #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 *, ...);
···
 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);