qcustomplot: Fix clang warnings: overrides a member function but is not marked 'override'

[marco-m]

This happens with clang on at least OS X Sierra: tons of:

includes/contrib/qcustomplot.h:3301:18: warning:
     'selectTest' overrides a member function but is not marked 'override'
     [-Winconsistent-missing-override]

I understand this is 3rd party code so you might have preferred using the -Wno-inconsistent-missing-override at compile-time and leave the file un-modified, but since the warnings are in the header, one would have to disable the warnings for all the QSyncthingTray source files including this header. So, I went for the approach of adding the missing Q_DECL_OVERRIDE to the header file. If upstream fixes it, it will fix it in the same way, so there should be no conflicts.

This fix allows to build on OS X with 0 warnings, which is a good thing :-)

[sieren]

@marco-m Thanks a lot for this! They annoyed me quite a bit as well. I originally planned on writing a few macros I'd put around the qcustomplot.h header include, but this will do for now :)
Thanks again! 👍

[sieren]

Added an issue for this so I won't forget. Will merge once CI goes green.

[marco-m]

Happy you like it!