Refactor: use <<= bit shift assignment operator in wxPGProperty::GetFlagsAsString.

Use it instead of << operator which is used to just shift bits in one variable.

src/propgrid/property.cpp

@@ -1904,7 +1904,7 @@ wxString wxPGProperty::GetFlagsAsString( FlagType flagsMask ) const
                 s << wxS("|");
             s << fs;
         }
-        a = a << 1;
+        a <<= 1;
     }
 
     return s;