output/solaris: include sys/stropts.h only on Solaris

This header had been available for a long time on Linux, but was
removed in glibc 2.30.  This commit moves the `#include` line inside
the `#ifdef __sun` block and adds a fake declaration of `I_FLUSH` for
the Linux build.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/630

src/output/plugins/SolarisOutputPlugin.cxx

@@ -22,7 +22,6 @@
 #include "system/FileDescriptor.hxx"
 #include "system/Error.hxx"
 
-#include <sys/stropts.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <unistd.h>
@@ -31,11 +30,18 @@
 
 #ifdef __sun
 #include <sys/audio.h>
+#include <sys/stropts.h>
 #else
 
 /* some fake declarations that allow build this plugin on systems
    other than Solaris, just to see if it compiles */
 
+#include <sys/ioctl.h>
+
+#ifndef I_FLUSH
+#define I_FLUSH 0
+#endif
+
 #define AUDIO_GETINFO 0
 #define AUDIO_SETINFO 0
 #define AUDIO_ENCODING_LINEAR 0