Fix Makefile overrides warnings

Both qserialport and qextserialport use the pattern of += into
PRIVATE_HEADERS and PUBLIC_HEADERS variables. Then at the end of the
.pri file they will += these into HEADERS. This leads to header files
being included more than once into the HEADERS variable. This in turn
causes QMake to generate a makefile which has the same target listed
more than once leading to the warning. Changing to *= removes the
duplicates. I checked the current qextserialport version and the
problem is still there. So upgrading will not fix. Not great to change
external lib .pri file, but it does fix the problem.
Fix overridden virtual function in qwt

QwtPlotZoomer::move hides QwtPlotPicker::move. The latest versions of
Qwt has this fixed. Pulling over a full new version seems dangerous
without extensive testing. Instead I pulled across the changes to just
this method from the latest version.
Change QwtPlotZoomer::move call to moveTo

Missed a call to QwtPlotZoomer::move in the previous commit.
Fix overloaded virtual warnings in OSG

I checked the latest version of OSG and it looks to still have the same
overload issue. Added #pragma’s to silence the warning around the
overloaded methods. Nasty to have to modify an external lib, but it’s
fairly innocous since it’s pragma only.