[core] Fix performance-move-const-arg errors in header files

As reported by clang-tidy-8.

include/mbgl/style/image.hpp

@@ -47,7 +47,7 @@ public:
                 false,
                 std::move(stretchX),
                 std::move(stretchY),
-                std::move(content)) {}
+                content) {}
     Image(const Image&);
 
     std::string getID() const;

src/mbgl/text/glyph.hpp

@@ -76,8 +76,8 @@ public:
           vertical(vertical_),
           font(font_),
           scale(scale_),
-          rect(std::move(rect_)),
-          metrics(std::move(metrics_)),
+          rect(rect_),
+          metrics(metrics_),
           imageID(std::move(imageID_)),
           sectionIndex(sectionIndex_) {}