util: Remove designator initializer from ScheduleBatchPriority

Although no compiler appears to complain about it, these are
not valid for c++11.
(http://en.cppreference.com/w/cpp/language/aggregate_initialization says they're c++20)

The structure is defined as:

   struct sched_param {
       int sched_priority;
   };

So passing 0 for the first field has the same effect.

src/util.cpp

@@ -1064,7 +1064,7 @@ fs::path AbsPathForConfigVal(const fs::path& path, bool net_specific)
 int ScheduleBatchPriority(void)
 {
 #ifdef SCHED_BATCH
-    const static sched_param param{.sched_priority = 0};
+    const static sched_param param{0};
     if (int ret = pthread_setschedparam(pthread_self(), SCHED_BATCH, &param)) {
         LogPrintf("Failed to pthread_setschedparam: %s\n", strerror(errno));
         return ret;