libcc1: use GCC_FE_VERSION_1 in C++ plugin

The C++ plugin defaults to version 0 of the base compiler API.
However, this is a mistake -- version 1 was introduced before the C++
API was even implemented.  This switches the default to version 1.
Note that the compiler-side plugin will accept this version, so it
should remain compatible.

libcc1

	* libcp1.cc (vtable): Use GCC_FE_VERSION_1.