Fix build error with glibc 2.26 and above

SIGUNUSED has been removed and causes compilation errors. Based on
the name it was probaably never used anyway, so just remove it.

reckless/src/crash_handler.cpp

@@ -41,7 +41,7 @@ std::initializer_list<int> const signals = {
     // SUSv2 + POSIX.1-2001 signals
     SIGBUS, SIGPOLL, SIGPROF, SIGSYS, SIGTRAP, SIGVTALRM, SIGXCPU, SIGXFSZ,
     // Various other signals
-    SIGIOT, SIGSTKFLT, SIGIO, SIGPWR, SIGUNUSED,
+    SIGIOT, SIGSTKFLT, SIGIO, SIGPWR,
     };
 
 std::vector<basic_log*> g_logs;