mixer/alsa: use BlockingCall() instead of EventLoop::AddCall()

This is safer, and works without epoll().  Fixes a build failure with
uClibc, which does not support epoll().

NEWS

@@ -4,6 +4,8 @@ ver 0.18.6 (not yet released)
 * output
   - openal: fix build failure on Mac OS X
   - osx: fix build failure
+* mixer
+  - alsa: fix build failure with uClibc
 * accept files without metadata
 
 ver 0.18.5 (2013/11/23)

src/mixer/AlsaMixerPlugin.cxx

@@ -24,6 +24,7 @@
 #include "Main.hxx"
 #include "event/MultiSocketMonitor.hxx"
 #include "event/Loop.hxx"
+#include "event/Call.hxx"
 #include "util/ASCII.hxx"
 #include "util/ReusableArray.hxx"
 #include "util/Error.hxx"
@@ -46,7 +47,7 @@ class AlsaMixerMonitor final : private MultiSocketMonitor {
 public:
 	AlsaMixerMonitor(EventLoop &_loop, snd_mixer_t *_mixer)
 		:MultiSocketMonitor(_loop), mixer(_mixer) {
-		_loop.AddCall([this](){ InvalidateSockets(); });
+		BlockingCall(_loop, [this](){ InvalidateSockets(); });
 	}
 
 private: