Fix LP64 builds after OpenBSD string changes.

Change-Id: I07202f6484e716d153d0387fcfc023e119438251
diff --git a/libc/arch-x86_64/x86_64.mk b/libc/arch-x86_64/x86_64.mk
index 9171c50..9bce065 100644
--- a/libc/arch-x86_64/x86_64.mk
+++ b/libc/arch-x86_64/x86_64.mk
@@ -4,7 +4,7 @@
     bionic/index.cpp \
     bionic/memchr.c \
     bionic/memcmp.c \
-    bionic/memcpy.c \
+    bionic/memcpy.cpp \
     bionic/memmove.c \
     bionic/memrchr.c \
     bionic/memset.c \