Fixed a signed-unsigned comparison in spatial_db.cc -- issue XRPLF#293

Summary:
Fixed a signed-unsigned comparison in spatial_db.cc

utilities/spatialdb/spatial_db.cc:542:38: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
cc1plus: all warnings being treated as errors
make: *** [utilities/spatialdb/spatial_db.o] Error 1

Test Plan:
make spatial_db_test
./spatial_db_test

Reviewers: ljin, sdong, reddragon, igor

Reviewed By: reddragon

Subscribers: reddragon, leveldb

Differential Revision: https://reviews.facebook.net/D23565

utilities/spatialdb/spatial_db.cc

@@ -517,7 +517,7 @@ class SpatialDBImpl : public SpatialDB {
       return Status::InvalidArgument("Spatial indexes can't be empty");
     }
 
-    const int kWriteOutEveryBytes = 1024 * 1024;  // 1MB
+    const size_t kWriteOutEveryBytes = 1024 * 1024;  // 1MB
     uint64_t id = next_id_.fetch_add(1);
 
     for (const auto& si : spatial_indexes) {