Add emplace_back to C++ vector declarations (GH-4527)

This definition could be improved when variadic templates are supported at some point, but since the types are not really relevant for Cython it should be fine to use C variadics here (especially since I do not expect that variadic templates will be supported anytime soon).

Closes https://github.com/cython/cython/issues/2171

Cython/Includes/libcpp/vector.pxd

@@ -86,3 +86,5 @@ cdef extern from "<vector>" namespace "std" nogil:
         T* data()
         const T* const_data "data"()
         void shrink_to_fit() except +
+        iterator emplace(const_iterator, ...) except +
+        T& emplace_back(...) except +