Reapply warning fix from previous S2 vendor to new S2 version (#269)

Author: paleolimbot

src/s2/s2region_coverer.h

@@ -272,7 +272,7 @@ class S2RegionCoverer {
     S2Cell cell;
     bool is_terminal;        // Cell should not be expanded further.
     int num_children = 0;    // Number of children that intersect the region.
-    Candidate* children[0];  // Actual size may be 0, 4, 16, or 64 elements.
+    __extension__ Candidate* children[0];  // Actual size may be 0, 4, 16, or 64 elements.
   };
 
   // If the cell intersects the given region, return a new candidate with no