build: Enable introspection during distcheck
build: Enable introspection during distcheck

Introspection builds just fine with srcdir != builddir since
e149fc4b5ff47164cf4e1cac01c4184b77bbd0e5

https://bugzilla.gnome.org/show_bug.cgi?id=759571

ritzk/gnome-online-accounts,ritzk/gnome-online-accounts,ritzk/gnome-online-accounts