build-sys: Disable deprecated warnings

There's not much point in having -Wdeprecated-declarations in this
version of BlueZ. Right now it's throwing the following warning:

audio/gstavdtpsink.c: In function 'gst_avdtp_sink_finalize':
audio/gstavdtpsink.c:198:2: error: 'g_mutex_free' is deprecated (declared at /usr/include/glib-2.0/glib/deprecated/gthread.h:273) [-Werror=deprecated-declarations]
audio/gstavdtpsink.c: In function 'gst_avdtp_sink_init':
audio/gstavdtpsink.c:1049:2: error: 'g_mutex_new' is deprecated (declared at /usr/include/glib-2.0/glib/deprecated/gthread.h:271) [-Werror=deprecated-declarations]
cc1: all warnings being treated as errors
audio/gsta2dpsink.c: In function 'gst_a2dp_sink_finalize':
audio/gsta2dpsink.c:82:2: error: 'g_mutex_free' is deprecated (declared at /usr/include/glib-2.0/glib/deprecated/gthread.h:273) [-Werror=deprecated-declarations]
audio/gsta2dpsink.c: In function 'gst_a2dp_sink_init':
audio/gsta2dpsink.c:711:2: error: 'g_mutex_new' is deprecated (declared at /usr/include/glib-2.0/glib/deprecated/gthread.h:271) [-Werror=deprecated-declarations]
cc1: all warnings being treated as errors

These functions were already removed from upstream, so there's not much
point in applying a patch upstream and backporting it. So use
-Wno-deprecated-functions instead.