Add nullptr to libcpp declarations.

Cython/Includes/libcpp/__init__.pxd

 cdef extern from *:
     ctypedef bint bool
+    ctypedef void* nullptr_t
+    nullptr_t nullptr

Cython/Includes/libcpp/memory.pxd

-from libcpp cimport bool
+from libcpp cimport bool, nullptr_t, nullptr
 
 cdef extern from "<memory>" namespace "std" nogil:
-    ctypedef void* nullptr_t; # Should be decltype(nullptr)
 
     cdef cppclass unique_ptr[T]:
         unique_ptr()