Unbreak the build for Windows
Unbreak the build for Windows

1) This export trick is necessary to get the project build on Windows. This was
not caught by the CI previously as it was turned off (force success).

2) It is also how other modules handle the exporting in their module global
header files.

Change-Id: I64be4e99089c7cfcdfaeb90f5406e2a016e3c0e1
Reviewed-by: Sergey Belyashov <8871b4c2d635fca691211507eadbac3d0bc2695a@gmail.com>
Reviewed-by: Simo Fält <b598cfbc9cb80e3067479e5abd365cdf44c9b327@digia.com>
Reviewed-by: Denis Shienkov <6a9c747bd2ba446682324cf340d3b86f30cf4d55@gmail.com>

RoadWorksSoftware/QtSerialPort-OS2,kobolabs/qtserialport,RoadWorksSoftware/QtSerialPort-OS2,kobolabs/qtserialport