Use anonymous namespace for local linkage

Author: joto

src/logging.cpp

@@ -11,11 +11,15 @@
 
 #include <osmium/thread/util.hpp>
 
+namespace {
+
 thread_local unsigned int this_thread_num = 0;
 
 /// Global logger singleton
 logger the_logger{};
 
+} // anonymous namespace
+
 /// Access the global logger singleton
 logger &get_logger() noexcept { return the_logger; }