layers: Fix size_t-int cast warning in descriptor_sets

Discovered in VS2015.

Change-Id: I2a42209a9ee9bd0e8f3da21c0f1a613315d5a6ce
diff --git a/layers/descriptor_sets.cpp b/layers/descriptor_sets.cpp
index f7baeb0..6cc0ec9 100644
--- a/layers/descriptor_sets.cpp
+++ b/layers/descriptor_sets.cpp
@@ -40,7 +40,7 @@
         uint32_t insert_index = 0; // Track vector index where we insert element
         if (bindings_.empty() || binding_num > bindings_.back().binding) {
             bindings_.push_back(safe_VkDescriptorSetLayoutBinding(&p_create_info->pBindings[i]));
-            insert_index = bindings_.size() - 1;
+            insert_index = static_cast<uint32_t>(bindings_.size()) - 1;
         } else { // out-of-order binding number, need to insert into vector in-order
             auto it = bindings_.begin();
             // Find currently binding's spot in vector