Added default value to func declaration.

Author: Victor Maryama

pandas/_libs/tslibs/timedeltas.pxd

@@ -6,4 +6,4 @@ from numpy cimport int64_t
 cpdef parse_timedelta_string(object ts, object specified_unit=*)
 cdef int64_t cast_from_unit(object ts, object unit) except? -1
 cpdef int64_t delta_to_nanoseconds(delta) except? -1
-cpdef convert_to_timedelta64(object ts, object unit)
+cpdef convert_to_timedelta64(object ts, object unit=*)