If you want to instrument functions from libc then there's no need to mess around with dlsyms, try prepending __libc_ to the function you're interested in:

void *calloc (size_t count, size_t size)
{
     void *ptr = __libc_calloc(count, size);
     DEBUG("calloc(%d, %d) = %p\n", count, size, ptr);
     return ptr;
}