Fix delete/free mismatch in QtXmlPatterns

Fixes valgrind warning like this:
Mismatched free() / delete / delete []
  at: operator delete(void*) (vg_replace_malloc.c:387)
  by: QPatternist::Decimal::toString(double) (qdecimal.cpp:121)

Reason for the warning is that toString above calls qdtoa the result of
which should be released with free(), not delete.

Change-Id: Iff07f2d07a4d65c642167fd07b980c6bdd15c117
Sanity-Review: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>