QtSql: compile with GCC 7

GCC 7 warns about implicit fall-throughs now.

Fix by adding the missing comments.

Change-Id: I7383f47e690b6334ef69c9df745c2205247ca7d0
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Mark Brand <mabrand@mabrand.nl>

src/sql/kernel/qsqldriver.cpp

@@ -654,6 +654,7 @@ QString QSqlDriver::formatValue(const QSqlField &field, bool trimStrings) const
                 break;
             }
         }
+            // fall through
         default:
             r = field.value().toString();
             break;