Add include required to build with muslc libc

--- viewer/svutil.cpp	2012-03-03 12:53:33.000000000 +0100
+++ viewer/svutil.cpp	2015-05-28 17:51:46.917525843 +0200
@@ -39,6 +39,7 @@
 #include <string.h>
 #include <netdb.h>
 #include <sys/socket.h>
+#include <sys/select.h>
 #ifdef __linux__
 #include <sys/prctl.h>
 #endif