fix(logger): Wrong conversion function called

For some reason when passing some non-const strings to convert, the
convert(T&& arg) method was used instead of the one specialized for
strings.

This caused an error in clang because you can't pass objects with
non-trivial types to varargs functions.

The best solution I found was to just add a specialized function for
non-const strings.

include/components/logger.hpp

@@ -100,6 +100,7 @@ class logger {
   /**
    * Convert string
    */
+  const char* convert(string& arg) const;
   const char* convert(const string& arg) const;
 
   /**

src/components/logger.cpp

@@ -13,6 +13,10 @@ POLYBAR_NS
 /**
  * Convert string
  */
+const char* logger::convert(string& arg) const {
+  return arg.c_str();
+}
+
 const char* logger::convert(const string& arg) const {
   return arg.c_str();
 }