change errno.h to cerrno

The former is deprecated by C++14. It's also functionally the same.

From the standard:

19.4
The header<cerrno>is described in Table 43. Its contents are the same as
the POSIX header<errno.h>,except that errno shall be defined as a macro.
[Note: The intent is to remain in close alignment with the POSIX
standard.] A separate errno value shall be provided for each thread.

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

src/output/plugins/FifoOutputPlugin.cxx

@@ -28,8 +28,9 @@
 #include "Log.hxx"
 #include "open.h"
 
+#include <cerrno>
+
 #include <sys/stat.h>
-#include <errno.h>
 #include <unistd.h>
 
 class FifoOutput final : AudioOutput {

src/output/plugins/OssOutputPlugin.cxx

@@ -28,13 +28,13 @@
 #include "Log.hxx"
 
 #include <cassert>
+#include <cerrno>
 #include <iterator>
 #include <stdexcept>
 
 #include <sys/stat.h>
 #include <sys/ioctl.h>
 #include <fcntl.h>
-#include <errno.h>
 #include <stdlib.h>
 #include <unistd.h>
 

src/output/plugins/SolarisOutputPlugin.cxx

@@ -22,12 +22,13 @@
 #include "system/FileDescriptor.hxx"
 #include "system/Error.hxx"
 
+#include <cerrno>
+
 #include <sys/ioctl.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <unistd.h>
 #include <fcntl.h>
-#include <errno.h>
 
 #if defined(__sun)
 #include <sys/audio.h>