Fix performance-for-range-copy warnings

Bug: 30413223
Test: make with WITH_TIDY=1 DEFAULT_GLOBAL_TIDY_CHECKS=-*,performance*
Change-Id: I3ad102f2b0f971266d57488a3bd57d312f7ee3e6
diff --git a/libunwindstack/DwarfCfa.cpp b/libunwindstack/DwarfCfa.cpp
index 0fa1638..4ba8a4b 100644
--- a/libunwindstack/DwarfCfa.cpp
+++ b/libunwindstack/DwarfCfa.cpp
@@ -260,7 +260,7 @@
   }
 
   // Log any of the expression data.
-  for (const auto line : expression_lines) {
+  for (const auto& line : expression_lines) {
     log(indent + 1, "%s", line.c_str());
   }
   return true;