This fix was pulled across from current QSerialPort codebase. I looked
into pulling across newer version of full QSerialPort. While doing that
I discovered that the version we are using has already been hacked to
work with us. It was fairly involved to redo those hacks. So decided to
pull individual change across instead since our version is already not
clean. When we switch to Qt5 all this will go away.

qserialport and qextserialport use the pattern of += into
PUBLIC_HEADERS and PRIVATE_HEADERS. This leads to the same header being
included twice. Which in turn causes the rule for moc in the makefile
to be added twice, hence the warning. Using *= instead of += removes
the duplication.

qserialport and qextserialport use the pattern of += into
PUBLIC_HEADERS and PRIVATE_HEADERS. This leads to the same header being
included twice. Which in turn causes the rule for moc in the makefile
to be added twice, hence the warning. Using *= instead of += removes
the duplication.