Added missing BuildRequires

BuildRequires:  pkgconfig(accounts-qt5)
BuildRequires:  pkgconfig(signon-oauth2plugin)

Tested in my OBS project against sailfish:1.0.3.8 target.
https://build.merproject.org/package/show?package=buteo-sync-plugin-caldav-manual&project=home%3Atbr%3Asailfish