fix global keyframes with no elements

src/compiler/compile/css/Stylesheet.ts

@@ -217,6 +217,11 @@ class Atrule {
 				if (type === 'Identifier') {
 					if (name.startsWith('-global-')) {
 						code.remove(start, start + 8);
+						this.children.forEach((rule: Rule) => {
+							rule.selectors.forEach(selector => {
+								selector.used = true;
+							});
+						});
 					} else {
 						code.overwrite(start, end, keyframes.get(name));
 					}

test/css/samples/global-keyframes-with-no-elements/expected.css

@@ -0,0 +1 @@
+@keyframes why{0%{color:red}100%{color:blue}}

test/css/samples/global-keyframes-with-no-elements/input.svelte

@@ -0,0 +1,6 @@
+<style>
+	@keyframes -global-why {
+		0% { color: red; }
+		100% { color: blue; }
+	}
+</style>