gccrs: Change specifier of some forward declaration

gccrs: Change specifier of some forward declaration

Change storage specifier of some forward declaration to satisfy compiler
warning.

gcc/rust/ChangeLog:

	* hir/tree/rust-hir-full-decls.h (struct Crate): Change to
	class.
	(class Crate): Likewise.
	(struct StructField): Likewise.
	(class StructField): Likewise.
	(struct TupleField): Likewise.
	(class TupleField): Likewise.
	(struct TraitFunctionDecl): Likewise.
	(class TraitFunctionDecl): Likewise.
	* rust-session-manager.h (struct Crate): Likewise.
	(class Crate): Likewise.

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