tcp: fix build on Solaris

Add missing header.

From Claes Nästén via http://gnats.netbsd.org/56509

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1364>

subprojects/gst-plugins-base/gst/tcp/gstmultifdsink.c

@@ -122,6 +122,10 @@
 #include "gsttcpelements.h"
 #include "gstmultifdsink.h"
 
+#if !defined(FIONREAD) && defined(__sun)
+#include <sys/filio.h>
+#endif
+
 #define NOT_IMPLEMENTED 0
 
 GST_DEBUG_CATEGORY_STATIC (multifdsink_debug);