examples: un-rot 1606-platform-metal-clear

ShaderHandle lives in modules/gpu/types.sx; bare-type visibility is
non-transitive (0763), so the example imports it directly. Builds for
ios-sim again.

examples/1606-platform-metal-clear.sx

@@ -21,6 +21,7 @@
 #import "modules/build.sx";
 #import "modules/platform/api.sx";
 #import "modules/platform/uikit.sx";
+#import "modules/gpu/types.sx";
 #import "modules/gpu/api.sx";
 #import "modules/gpu/metal.sx";