[RocksDB] fix build

Summary: forgot to include signal_test.cc

Test Plan: make check

Reviewers: sheki

Reviewed By: sheki

CC: leveldb

Differential Revision: https://reviews.facebook.net/D10281

util/signal_test.cc

@@ -0,0 +1,27 @@
+#include "util/stack_trace.h"
+#include <assert.h>
+
+void f0() {
+  char *p = nullptr;
+  *p = 10;  /* SIGSEGV here!! */
+}
+
+void f1() {
+  f0();
+}
+
+void f2() {
+  f1();
+}
+
+void f3() {
+  f2();
+}
+
+int main() {
+  leveldb::InstallStackTraceHandler();
+
+  f3();
+
+  return 0;
+}