mdbx: fix minor MinGW warning.

ChangeLog.md

@@ -17,6 +17,7 @@ Minors:
  - Fixed an extra ensure/assertion check of `oldest_reader` inside `txn_end()`.
  - Removed description of deprecated usage of `MDBX_NODUPDATA`.
  - Fixed regression ASAN/Valgring-enabled builds.
+ - Fixed minor MingGW warning.
 
 
 -------------------------------------------------------------------------------

src/core.c

@@ -5244,7 +5244,7 @@ static txnid_t find_oldest_reader(MDBX_env *const env, const txnid_t steady) {
     new_oldest = steady;
 
     for (unsigned i = 0; i < snap_nreaders; ++i) {
-      const mdbx_pid_t pid =
+      const uint32_t pid =
           atomic_load32(&lck->mti_readers[i].mr_pid, mo_AcquireRelease);
       if (!pid)
         continue;