Make pulseaudio dependency private

Otherwise projects that depend on multimedia module
fail to build.

Change-Id: I44a614fd3b2bea934149f8bf55eaeb17069258d5
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

src/multimedia/audio/audio.pri

@@ -41,7 +41,7 @@ SOURCES += \
            audio/qaudiohelpers.cpp
 
 qtConfig(pulseaudio) {
-    QMAKE_USE += pulseaudio
+    QMAKE_USE_FOR_PRIVATE += pulseaudio
 
     DEFINES += QT_MULTIMEDIA_PULSEAUDIO
     PRIVATE_HEADERS += audio/qsoundeffect_pulse_p.h