[Positioning] TextFileWriter: fix text writing

We wrote a C string instead of a string into the file stream, which
caused valgrind errors.

owlps-positioning/src/textfilewriter.cc

@@ -40,6 +40,6 @@ bool TextFileWriter::write_text(const string &text)
   if (! file)
     return false ;
 
-  file << text.c_str() ;
+  file << text ;
   return true ;
 }