[build] Fix dbus-sharp.csproj compilation on Mono.
[build] Fix dbus-sharp.csproj compilation on Mono.

Unfortunately, xbuild seems to not like very much conditional defines. Fold NET35 constant definition in and instead rely on people using a separate configuration.

openmedicus/dbus-sharp,mono/dbus-sharp,Tragetaschen/dbus-sharp,mono/dbus-sharp,Tragetaschen/dbus-sharp,arfbtwn/dbus-sharp,arfbtwn/dbus-sharp,openmedicus/dbus-sharp