gccrs: Move default visitor templates to header

Move default ast visitor template implementation to headers in order to
match the codebase and avoid link errors.

gcc/rust/ChangeLog:

	* ast/rust-ast-visitor.cc (DefaultASTVisitor::visit): Move from here...
	* ast/rust-ast-visitor.h: ... to here.

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