[BUG] Fix crash during Cython build (#500)

python/dgl/_ffi/_cython/base.pxi

 from ..base import DGLError
 from libcpp.vector cimport vector
+from libcpp cimport bool
 from cpython.version cimport PY_MAJOR_VERSION
 from cpython cimport pycapsule
 from libc.stdint cimport int64_t, uint64_t, uint8_t, uint16_t
@@ -56,6 +57,8 @@ cdef extern from "dgl/runtime/c_runtime_api.h":
 
 ctypedef int64_t dgl_index_t
 ctypedef DLTensor* DLTensorHandle
+ctypedef DLTensor DGLArray
+ctypedef DGLArray* CDGLArrayHandle
 ctypedef void* DGLStreamHandle
 ctypedef void* DGLRetValueHandle
 ctypedef void* DGLFunctionHandle
@@ -101,7 +104,7 @@ cdef extern from "dgl/runtime/c_runtime_api.h":
                                int dtype_bits,
                                int dtype_lanes,
                                bool is_create,
-                               DGLArrayHandle* out)
+                               CDGLArrayHandle* out)
     int DGLArrayFree(DLTensorHandle handle)
     int DGLArrayCopyFromTo(DLTensorHandle src,
                            DLTensorHandle to,