Add missing placeholder to less import file

Author: jlukic

src/semantic.less

@@ -26,6 +26,7 @@
 & { @import "definitions/elements/label"; }
 & { @import "definitions/elements/list"; }
 & { @import "definitions/elements/loader"; }
+& { @import "definitions/elements/placeholder"; }
 & { @import "definitions/elements/rail"; }
 & { @import "definitions/elements/reveal"; }
 & { @import "definitions/elements/segment"; }