Don't rely on transitive #includes.

Change-Id: I76af535da581002b7c5315d432e5c19db62a86eb
diff --git a/src/thread.cc b/src/thread.cc
index 6fcbaca..3892c23 100644
--- a/src/thread.cc
+++ b/src/thread.cc
@@ -18,6 +18,7 @@
 
 #include <dynamic_annotations.h>
 #include <pthread.h>
+#include <signal.h>
 #include <sys/mman.h>
 #include <sys/resource.h>
 #include <sys/time.h>