[core] Include what you use

tmpsantos · tmpsantos · commit 375e31891de2 · 2017-01-20T20:24:11.000+01:00
diff --git a/include/mbgl/style/layer.hpp b/include/mbgl/style/layer.hpp
@@ -4,6 +4,7 @@
 #include <mbgl/style/types.hpp>
 
 #include <memory>
+#include <string>
 
 namespace mbgl {
 namespace style {
