g_steal_pointer: make it C++ clean

g_steal_pointer: make it C++ clean

We have a test that #includes our headers from a C++ program to make
sure we don't throw any errors or warnings as a result of that.

The new inline implementation of g_steal_pointer() does an implicit
conversion from (void *), which is not valid in C++.

Add a cast to avoid the problem.

Thanks to Ignacio Casal Quinteiro for the report.