Allow compiling with -Werror=strict-prototypes (#56)
Allow compiling with -Werror=strict-prototypes (#56)

Without this, with gcc 7.3.0, we see things like:

/xgboost/include/xgboost/c_api.h:98:1: error: function
declaration isn't a prototype [-Werror=strict-prototypes]
 XGB_DLL const char *XGBGetLastError();
  ^~~~~~~
dmlc/rabit,dmlc/xgboost,dmlc/rabit,dmlc/rabit,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/rabit,dmlc/xgboost,dmlc/xgboost