Silence makefile override warnings

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.

libs/serialport/qserialport.pri

@@ -55,4 +55,4 @@ unix:!symbian {
     }
 }
 
-HEADERS += $$PUBLIC_HEADERS $$PRIVATE_HEADERS
+HEADERS *= $$PUBLIC_HEADERS $$PRIVATE_HEADERS