Added constants for data types [skip ci]

Author: ankane

lib/lightgbm/dataset.rb

@@ -177,7 +177,7 @@ def construct
           c_data.write_array_of_double(flat_data)
         end
 
-        check_result FFI.LGBM_DatasetCreateFromMat(c_data, 1, nrow, ncol, 1, parameters, reference, handle)
+        check_result FFI.LGBM_DatasetCreateFromMat(c_data, FFI::C_API_DTYPE_FLOAT64, nrow, ncol, 1, parameters, reference, handle)
       end
       if used_indices
         @handle = handle.read_pointer

lib/lightgbm/ffi.rb

@@ -15,6 +15,11 @@ module FFI
     # https://github.com/microsoft/LightGBM/blob/master/include/LightGBM/c_api.h
     # keep same order
 
+    C_API_DTYPE_FLOAT32 = 0
+    C_API_DTYPE_FLOAT64 = 1
+    C_API_DTYPE_INT32 = 2
+    C_API_DTYPE_INT64 = 3
+
     C_API_PREDICT_NORMAL = 0
     C_API_PREDICT_RAW_SCORE = 1
     C_API_PREDICT_LEAF_INDEX = 2