QNX: Tidy up QMAKE_LFLAGS in qnx mkspecs

QNX: Tidy up QMAKE_LFLAGS in qnx mkspecs

We do not need to include -lang-c++ more than once in QMAKE_LFLAGS*
variables. The fact that this was not appearing more than once was due
to QMAKE_LFLAGS being overwritten (rather than appended to) in
qcc-base-qnx.conf and a weird coincindence in qmake's code to dedupe
libraries passed in to QMAKE_LFLAGS. The deduping was working based
upon options beginning with "-l", including the -lang-c++ option here.

Change-Id: I983f216c0e362a9fe6a924074c5d84aaa659a14f
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>