Add missing header inclusion

Header relied on transitive header inclusion.

gcc/rust/ChangeLog:

	* ast/rust-ast-formatting.h (RUST_AST_FORMATTING_H): Include required
	headers directly.

Signed-off-by: Pierre-Emmanuel Patry <[email protected]>

Author: P-E-P

gcc/rust/ast/rust-ast-formatting.h

@@ -19,6 +19,9 @@
 #ifndef RUST_AST_FORMATTING_H
 #define RUST_AST_FORMATTING_H
 
+#include "rust-ast.h"
+#include "rust-system.h"
+
 namespace Rust {
 namespace AST {