Fix llvm signal tests build.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375406 91177308-0d34-0410-b5e6-96231b3b80d8

unittests/Support/SignalsTest.cpp

@@ -9,6 +9,7 @@
 #if !defined(_WIN32)
 #include <unistd.h>
 #include <sysexits.h>
+#include <signal.h>
 #endif // !defined(_WIN32)
 
 #include "llvm/Support/Signals.h"