Add DISABLE_WARNINGS Macro for QCustomPlot

[sieren]

QCustomPlot isn't using C++11 thus we're getting a few warnings about missing overrides. This has been fixed in https://github.com/sieren/QSyncthingTray/pull/188 however an update to the upstream will kill this. Thus adding a cross-compiler macro (msvc, gcc, clang) around the header include should be added at some point.