test: Add missing signal.h header

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

Reported by denis2342 on IRC.

Github-Pull: bitcoin#12447
Rebased-From: dd7e42c
Tree-SHA512: 10ead029bb59f5d69e37b5679c710f22d64051de26e1ec8342eec4e4dec4d76249e16dff78d192972bcb8d139d99c7555a7cb2fe43b2b911103eab6d6f943b79

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