readsbrrd: cast timespec.tv_sec addend to time_t

This fixes the following compile error on systems using musl libc:

readsbrrd.c: In function 'main':
readsbrrd.c:581:23: error: '__time_t' undeclared (first use in this function); did you mean 'time_t'?
  581 |         ts.tv_sec += (__time_t) (rrd.step * 1.5);
      |                       ^~~~~~~~
      |                       time_t

Compile-tested on both glibc and musl systems.

Signed-off-by: Stijn Tintel <[email protected]>

readsbrrd.c

@@ -578,7 +578,7 @@ int main(int argc, char** argv) {
     // Run this until we get a termination signal.
     while (!readsbrrd_exit) {
         clock_gettime(CLOCK_REALTIME, &ts);
-        ts.tv_sec += (__time_t) rrd.step * 1.5;
+        ts.tv_sec += (time_t) rrd.step * 1.5;
         r = sem_getvalue(stats_semptr, &semcnt);
         // Avoid frequent updates when more than one event is queued in semaphore.
         // Update only one very last event.