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.