layers: Fixed portablity issue in new ext support

Added default constructor for LAST_BOUND_STATE because the default
constructor VS2013 was calling for unordered_map::operator [] was
leaving the structure uninitialized.

Change-Id: I7d74b42f762f0ee7972fa8ad381f7aec502c19ed
diff --git a/layers/core_validation_types.h b/layers/core_validation_types.h
index 255c38e..4a4de68 100644
--- a/layers/core_validation_types.h
+++ b/layers/core_validation_types.h
@@ -777,6 +777,7 @@
 
 // Track last states that are bound per pipeline bind point (Gfx & Compute)
 struct LAST_BOUND_STATE {
+    LAST_BOUND_STATE() { reset(); }  // must define default constructor for portability reasons
     PIPELINE_STATE *pipeline_state;
     VkPipelineLayout pipeline_layout;
     // Track each set that has been bound