output/pulse: don't expose internal struct in public header

Provide _lock() and _unlock() to wrap all accesses from the mixer
plugin.

test/read_mixer.c

@@ -33,6 +33,16 @@
 #ifdef HAVE_PULSE
 #include "output/pulse_output_plugin.h"
 
+void
+pulse_output_lock(G_GNUC_UNUSED struct pulse_output *po)
+{
+}
+
+void
+pulse_output_unlock(G_GNUC_UNUSED struct pulse_output *po)
+{
+}
+
 void
 pulse_output_set_mixer(G_GNUC_UNUSED struct pulse_output *po,
 		       G_GNUC_UNUSED struct pulse_mixer *pm)