test: Add missing signal.h header

util_tests.cpp needs to include the signal.h header on FreeBSD.

Reported by denis2342 on IRC.

Author: laanwj

src/test/util_tests.cpp

@@ -14,6 +14,7 @@
 #include <stdint.h>
 #include <vector>
 #ifndef WIN32
+#include <signal.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 #endif