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/encoder/plugins/LameEncoderPlugin.cxx

@@ -26,9 +26,9 @@
 
 #include <lame/lame.h>
 
+#include <cassert>
 #include <stdexcept>
 
-#include <assert.h>
 #include <string.h>
 
 class LameEncoder final : public Encoder {

src/encoder/plugins/OpusEncoderPlugin.cxx

@@ -26,9 +26,9 @@
 #include <opus.h>
 #include <ogg/ogg.h>
 
+#include <cassert>
 #include <stdexcept>
 
-#include <assert.h>
 #include <stdlib.h>
 
 namespace {

src/encoder/plugins/TwolameEncoderPlugin.cxx

@@ -27,9 +27,9 @@
 
 #include <twolame.h>
 
+#include <cassert>
 #include <stdexcept>
 
-#include <assert.h>
 #include <string.h>
 
 class TwolameEncoder final : public Encoder {

src/encoder/plugins/WaveEncoderPlugin.cxx

@@ -22,7 +22,8 @@
 #include "util/ByteOrder.hxx"
 #include "util/DynamicFifoBuffer.hxx"
 
-#include <assert.h>
+#include <cassert>
+
 #include <string.h>
 
 static constexpr uint16_t WAVE_FORMAT_PCM = 1;