gtk3, gtk4: Use correct scss syntax instead of sass syntax in a scss file

This caused build failures, e.g. as reported in !47 (comment 421238)

src/gtk3/widgets/_entry.scss

@@ -210,7 +210,7 @@ treeview entry {
 
 combobox > box > button.combo:not(:only-child) {
 	background: none;
-	@include entry(normal)
+	@include entry(normal);
 
 	&:focus {
 		@include entry(focus);

src/gtk4/widgets/_entry.scss

@@ -210,7 +210,7 @@ treeview entry {
 
 combobox > box > button.combo:not(:only-child) {
 	background: none;
-	@include entry(normal)
+	@include entry(normal);
 
 	&:focus {
 		@include entry(focus);