BUILD: log: iovec requires to include sys/uio.h on OpenBSD

The following commit merged into 1.6-dev6 broke the build on OpenBSD :

  609ac2a ("MEDIUM: log: replace sendto() with sendmsg() in __send_log()")

Including sys/uio.h is enough to fix this. This fix needs to be backported
to 1.6.

src/log.c

@@ -22,6 +22,7 @@
 #include <errno.h>
 
 #include <sys/time.h>
+#include <sys/uio.h>
 
 #include <common/config.h>
 #include <common/compat.h>