Fix -Wrange-loop-construct range var copy warning

Change range loop variable (and subsequent field reference) to
references to avoid copy (and copy warning).

Change-Id: I81255f3c0c787256bb208de0581a375b3b12b383
diff --git a/layers/synchronization_validation.cpp b/layers/synchronization_validation.cpp
index e51c5d1..854bff3 100644
--- a/layers/synchronization_validation.cpp
+++ b/layers/synchronization_validation.cpp
@@ -170,8 +170,8 @@
                                            std::map<VkPipelineStageFlagBits, VkPipelineStageFlags> &map) {
     VkPipelineStageFlags unscanned = stage_mask;
     VkPipelineStageFlags related = 0;
-    for (const auto entry : map) {
-        const auto stage = entry.first;
+    for (const auto &entry : map) {
+        const auto &stage = entry.first;
         if (stage & unscanned) {
             related = related | entry.second;
             unscanned = unscanned & ~stage;