Fix building with gcc-47

https://bugs.gentoo.org/show_bug.cgi?id=423045

Patch written by Kacper Kowalik <xarthisius@gentoo.org>
--- a/valknut/main.cpp
+++ b/valknut/main.cpp
@@ -22,6 +22,7 @@
 #endif
 
 #include <stdlib.h>
+#include <unistd.h>   /* _exit */
 #ifndef WIN32
 #include <signal.h>
 #endif