fix compilation with gcc-4.7 (ticket #3478)

thanks for strcat for the patch.

src/visualizer.cpp

@@ -31,6 +31,7 @@
 #include <limits>
 #include <fcntl.h>
 #include <sys/time.h>
+#include <unistd.h>
 
 using Global::MainStartY;
 using Global::MainHeight;