fix build regression in oldmalloc

commit 8d37958d58cf36f53d5fcc7a8aa6d633da6071b2 inadvertently broke
oldmalloc by having it implement __libc_malloc rather than
__libc_malloc_impl.

src/malloc/oldmalloc/malloc.c

@@ -11,7 +11,7 @@
 #include "malloc_impl.h"
 #include "fork_impl.h"
 
-#define malloc __libc_malloc
+#define malloc __libc_malloc_impl
 #define realloc __libc_realloc
 #define free __libc_free