build-sys: meson: Require bluez dependency if bluez5 feature is enabled

Build breaks if bluez5 and bluez5-native-headset are both enabled
but bluez headers are not available.

Fix this by changing `bluez5` to Meson feature requiring `bluez` dependency.

Part-of: <pulseaudio/pulseaudio!583>
2 jobs for pr/meson-check-bluetooth-header in 2 minutes and 25 seconds (queued for 8 seconds)