output/httpd: include sys/types.h

On Mac OS X, the httpd plugin cannot be compiled, because OS X's
system headers do nto include sys/types.h, although they use
u_int32_t.

src/output/httpd_output_plugin.c

@@ -28,6 +28,7 @@
 
 #include <assert.h>
 
+#include <sys/types.h>
 #include <netinet/in.h>
 #include <netdb.h>
 #include <unistd.h>