Added <unistd.h> include reported to fix a Solaris build.

src/lib/util/io/socketsession.cc

@@ -14,6 +14,8 @@
 
 #include <config.h>
 
+#include <unistd.h>
+
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <sys/uio.h>