text_formatter: Avoid unnecessary string temporary creation in PrintMessage()

operator+ for std::string creates an entirely new string, which is kind
of unnecessary here if we just want to append a null terminator to the
existing one.

Reduces the total amount of potential allocations that need to be done
in the logging path.

src/common/logging/text_formatter.cpp

@@ -31,7 +31,7 @@ std::string FormatLogMessage(const Entry& entry) {
 }
 
 void PrintMessage(const Entry& entry) {
-    auto str = FormatLogMessage(entry) + '\n';
+    const auto str = FormatLogMessage(entry).append(1, '\n');
     fputs(str.c_str(), stderr);
 }