replace assert.h with cassert

The former was deprecated with C++14.

According to the C++11 and C++17 standards, both files are identical.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/mixer/plugins/OssMixerPlugin.cxx

@@ -26,7 +26,8 @@
 #include "util/RuntimeError.hxx"
 #include "Log.hxx"
 
-#include <assert.h>
+#include <cassert>
+
 #include <string.h>
 #include <sys/ioctl.h>
 #include <fcntl.h>

src/mixer/plugins/PulseMixerPlugin.cxx

@@ -32,10 +32,9 @@
 #include <pulse/stream.h>
 #include <pulse/subscribe.h>
 
+#include <cassert>
 #include <stdexcept>
 
-#include <assert.h>
-
 class PulseMixer final : public Mixer {
 	PulseOutput &output;
 

src/mixer/plugins/SoftwareMixerPlugin.cxx

@@ -22,7 +22,8 @@
 #include "filter/plugins/VolumeFilterPlugin.hxx"
 #include "pcm/Volume.hxx"
 
-#include <assert.h>
+#include <cassert>
+
 #include <math.h>
 
 class SoftwareMixer final : public Mixer {

src/mixer/plugins/WinmmMixerPlugin.cxx

@@ -23,9 +23,9 @@
 
 #include <mmsystem.h>
 
+#include <cassert>
 #include <stdexcept>
 
-#include <assert.h>
 #include <math.h>
 #include <windows.h>