bustage fix for old gcc; already built on speedracer to ensure that it works, and on Mac to ensure that doesn't break modern compilers

git-svn-id: svn://10.0.0.236/trunk@78083 18797224-902f-48f8-a5cc-f745e15eee43

mozilla/xpcom/string/obsolete/nsString.h

@@ -544,6 +544,8 @@ public:
     char mBuffer[kDefaultStringSize];
 };
 
+NS_DEF_DERIVED_STRING_OPERATOR_PLUS(nsCAutoString, char)
+
 /**
  * A helper class that converts a UCS2 string to UTF8
  */