tree-wide: make constant ratelimit compound actually const

The compiler should recognize that these are constant expressions, but
let's better make this explicit, so that the linker can safely share the
initializations all over the place.

src/basic/fs-util.c

@@ -903,7 +903,7 @@ int posix_fallocate_loop(int fd, uint64_t offset, uint64_t size) {
 
         /* On EINTR try a couple of times more, but protect against busy looping
          * (not more than 16 times per 10s) */
-        rl = (RateLimit) { 10 * USEC_PER_SEC, 16 };
+        rl = (const RateLimit) { 10 * USEC_PER_SEC, 16 };
         while (ratelimit_below(&rl)) {
                 r = posix_fallocate(fd, offset, size);
                 if (r != EINTR)