Fix build after y2038 changes in glib.

SIOCGSTAMP is defined in linux/sockios.h, not asm/sockios.h now.
diff --git a/src/dnsmasq.h b/src/dnsmasq.h
index ff3204a..3ef04ad 100644
--- a/src/dnsmasq.h
+++ b/src/dnsmasq.h
@@ -137,6 +137,7 @@
 #endif
 
 #if defined(HAVE_LINUX_NETWORK)
+#include <linux/sockios.h>
 #include <linux/capability.h>
 /* There doesn't seem to be a universally-available 
    userspace header for these. */