Fix mac build

Change-Id: Ia3da3cf0d191ace07cb9f883943c093278856dbd
diff --git a/src/base/unix_socket_unittest.cc b/src/base/unix_socket_unittest.cc
index 06dd63a..531f1e4 100644
--- a/src/base/unix_socket_unittest.cc
+++ b/src/base/unix_socket_unittest.cc
@@ -16,6 +16,7 @@
 
 #include "perfetto/base/unix_socket.h"
 
+#include <signal.h>
 #include <sys/mman.h>
 
 #include <list>