BUG/MINOR: contrib/mod_defender: build fix

In similar manner than modsecurity, making the build possible under *BSD flavors, the -lm for ceilf function.
diff --git a/contrib/mod_defender/spoa.h b/contrib/mod_defender/spoa.h
index 332f387..726007f 100644
--- a/contrib/mod_defender/spoa.h
+++ b/contrib/mod_defender/spoa.h
@@ -16,6 +16,7 @@
 #define __SPOA_H__
 
 #include <sys/time.h>
+#undef LIST_HEAD
 
 #include <event2/util.h>
 #include <event2/event.h>