IWYU fixes for sigchainlib/sigchain.cc

This change adds the necessary headers for std::initializer_list, errno,
and strerror.

Bug: None
Test: m
Change-Id: I9b97635f378a205604c75601cddd6f87883cdb4b
diff --git a/sigchainlib/sigchain.cc b/sigchainlib/sigchain.cc
index 1727f88..cc1b78d 100644
--- a/sigchainlib/sigchain.cc
+++ b/sigchainlib/sigchain.cc
@@ -22,10 +22,13 @@
 #endif
 
 #include <dlfcn.h>
+#include <errno.h>
 #include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 
+#include <initializer_list>
 #include <utility>
 
 #include "sigchain.h"