[PRISM] Fix compiler warning for min_tmp_hash_length

prism_compile.c:1406:27: warning: comparison of integer expressions of different signedness: ‘size_t’ {aka ‘long unsigned int’} and ‘int’ [-Wsign-compare]
 1406 |                 if (count >= min_tmp_hash_length) {
      |                           ^~

prism_compile.c

@@ -1360,7 +1360,7 @@ pm_compile_hash_elements(rb_iseq_t *iseq, const pm_node_t *node, const pm_node_l
     // by newhash or hash merge). Double splat nodes should be merged using the
     // merge_kwd method call.
     const int max_stack_length = 0x100;
-    const int min_tmp_hash_length = 0x800;
+    const unsigned int min_tmp_hash_length = 0x800;
 
     int stack_length = 0;
     bool first_chunk = true;