gccrs: Move callbacked function in a new namespace

Move the function registered as a callback in a new anonymous namespace
in order to group every callback in the future under the same namespace.

gcc/rust/ChangeLog:

	* expand/rust-proc-macro.cc: Remove static modifier and move to
	an anonymous namespace.

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

gcc/rust/expand/rust-proc-macro.cc

@@ -26,7 +26,9 @@ namespace Rust {
 
 const std::string PROC_MACRO_DECL_PREFIX = "__gccrs_proc_macro_decls_";
 
-static ProcMacro::TokenStream
+namespace {
+
+ProcMacro::TokenStream
 tokenstream_from_string (std::string &data, bool &lex_error)
 {
   // FIXME: Insert location pointing to call site in tokens
@@ -56,6 +58,8 @@ static_assert (
 	       ProcMacro::from_str_function_t>::value,
   "Registration callback signature not synced, check proc macro internals.");
 
+} // namespace
+
 template <typename Symbol, typename Callback>
 bool
 register_callback (void *handle, Symbol, std::string symbol_name,