[GCC-XML]Explicit template instantiation

[Brad King]

> C:\type test.h
> #include <vector>
> template class std::vector<double>;
>
> C:\gccxml -fxml=test.xml test.h

Try writing the code in test.cxx.  The internals of GCC-XML invoke the gcc
c++ parser executable, which may not identify a .h file as C++.  If the .h
is included in another C++ file, then it should work fine:

foo.cxx:
#include "test.h"

gccxml -fxml=test.xml foo.cxx

-Brad