undef mmap64 function

mmap64 can already been defined and as a result the following error is
raised:

In file included from src/malloc_hook.cc:686:0:
src/malloc_hook_mmap_linux.h: In function ‘void* mmap(void*, size_t, int, int, int, off_t)’:
src/malloc_hook_mmap_linux.h:173:18: error: redefinition of ‘void* mmap(void*, size_t, int, int, int, off_t)’
 extern "C" void* mmap(void *start, size_t length, int prot, int flags,
                  ^~~~
In file included from src/malloc_hook.cc:41:0:
src/malloc_hook_mmap_linux.h:159:18: note: ‘void* mmap(void*, size_t, int, int, int, off_t)’ previously defined here
 extern "C" void* mmap64(void *start, size_t length, int prot, int flags,
                  ^
Makefile:4874: recipe for target 'src/libtcmalloc_minimal_internal_la-malloc_hook.lo' failed

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
1 file changed