iovec-util: make IOVEC_INCREMENT a regular function too

Even more than with the previous commit, this is not a trivial function
and there's no reason to believe this will actually be inlined nor that
it would be beneficial.

src/basic/log.c

@@ -541,7 +541,7 @@ static int write_to_syslog(
                 if (!syslog_is_stream)
                         break;
 
-                if (IOVEC_INCREMENT(iovec, ELEMENTSOF(iovec), n))
+                if (iovec_increment(iovec, ELEMENTSOF(iovec), n))
                         break;
         }