qt: fix alsa-lib dependency

If BR2_PACKAGE_QT_AUDIO_BACKEND=y, QtMultimedia requires the audio backend
to be already in staging in order to compile.
The backend is system-dependent, but since buildroot builds Linux systems
only, it is safe to assume that the it will always be ALSA.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/qt/Config.in

@@ -311,8 +311,9 @@ config BR2_PACKAGE_QT_MULTIMEDIA
 config BR2_PACKAGE_QT_AUDIO_BACKEND
 	bool "QtMultimedia Audio backend"
 	depends on BR2_PACKAGE_QT_MULTIMEDIA
+	select BR2_PACKAGE_ALSA_LIB
 	help
-	  Build the platform audio backend into QtMultimedia if available
+	  Build the ALSA audio backend into QtMultimedia
 
 config BR2_PACKAGE_QT_SVG
 	bool "SVG Module"

package/qt/qt.mk

@@ -312,6 +312,7 @@ endif
 
 ifeq ($(BR2_PACKAGE_QT_AUDIO_BACKEND),y)
 QT_CONFIGURE+= -audio-backend
+QT_DEP_LIBS+=alsa-lib
 else
 QT_CONFIGURE+= -no-audio-backend
 endif