gccrs: libproc_macro: Change rust Ident definition

Change rust interface Ident definition to match C++ struct definition.
This structure now uses an FFIString instead of raw pointers.

libgrust/ChangeLog:

	* libproc_macro/rust/bridge/ident.rs: Change raw
	pointer string with an FFIString.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>