parser,markused: support @[markused] for interface declarations too (#24963)

vlib/v/ast/types.v

@@ -208,6 +208,7 @@ pub mut:
 	conversions shared map[int][]Type
 	// generic interface support
 	is_generic     bool
+	is_markused    bool
 	generic_types  []Type
 	concrete_types []Type
 	parent_type    Type

vlib/v/markused/walker.v

@@ -169,6 +169,8 @@ pub fn (mut w Walker) mark_markused_syms() {
 	for sym in w.table.type_symbols {
 		if sym.info is ast.Struct && sym.info.is_markused {
 			w.mark_by_sym(sym)
+		} else if sym.info is ast.Interface && sym.info.is_markused {
+			w.mark_by_sym(sym)
 		}
 	}
 }

vlib/v/parser/struct.v

@@ -640,6 +640,7 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 		info:     ast.Interface{
 			types:         []
 			is_generic:    generic_types.len > 0
+			is_markused:   attrs.contains('markused')
 			generic_types: generic_types
 		}
 		language: language

vlib/v/tests/skip_unused/interface_struct_unused.vv

@@ -0,0 +1,7 @@
+@[markused]
+struct Foo {}
+
+@[markused]
+interface Bar {
+	a Foo
+}