replace pthread_yield with POSIX sched_yield

[9000h]

it's form the BSD port https://linux.die.net/man/3/pthread_yield

--- a/PLUGINS/src/softhddevice/audio.c
+++ b/PLUGINS/src/softhddevice/audio.c
@@ -48,6 +48,7 @@
 #include <inttypes.h>
 #include <string.h>
 #include <math.h>
+#include <sched.h>

 #include <libintl.h>
 #define _(str) gettext(str)            ///< gettext shortcut
@@ -1584,7 +1585,7 @@
        if (err < 0) {                  // underrun error
            return -1;
        }
-       pthread_yield();
+       sched_yield();
        usleep(OssFragmentTime * 1000); // let fill/empty the buffers
        return 0;
     }