Define realpath for MinGW builds

Define realpath for MinGW builds

- Define realpath the same way as in win32/rrd_config.h
- Fixes the following compiler warnings in rrd_client.c:
  115:11: warning: implicit declaration of function 'realpath';
  [-Wimplicit-function-declaration] ret = realpath(path, NULL);
  115:11: warning: nested extern declaration of 'realpath'
  [-Wnested-externs]
  115:9: warning: assignment to 'char *' from 'int' makes pointer from
  integer without a cast [-Wint-conversion] ret = realpath(path, NULL);
  132:22: warning: assignment to 'char *' from 'int' makes pointer from
  integer without a cast [-Wint-conversion]
  dir_path = realpath(dir, NULL);