issue doxygen#10928 Setting JAVACC_CHAR_TYPE

`JAVACC_CHAR_TYPE` was already set in the main `CMakeLists.txt`  but its value was not used here.

src/qcstring.h

@@ -92,7 +92,7 @@ inline int qstricmp_sort( const char *str1, const char *str2 )
 
 int qstrnicmp( const char *str1, const char *str2, size_t len );
 
-using JavaCCString = std::basic_string<unsigned char>;
+using JavaCCString = std::basic_string<JAVACC_CHAR_TYPE>;
 
 /** This is an alternative implementation of QCString. It provides basically
  *  the same functions but uses std::string as the underlying string type