cgen: emit forward typedefs for struct types used in C extern function signatures

When -skip_unused is active, struct types referenced only in extern function
declarations may not get their typedefs emitted, causing 'unknown type name'
errors in the generated C. This restores the forward-declaration logic that
was removed in 43fdf0f, ensuring types like main__Iwad_t are typedef'd
before the extern prototypes that reference them (fixes C2V apps CI).

Author: medvednikov

vlib/v/gen/c/fn.v

@@ -675,6 +675,44 @@ fn (g &Gen) should_emit_c_extern_decl(node ast.FnDecl) bool {
 	return false
 }
 
+fn (mut g Gen) ensure_extern_sig_type_decls(node ast.FnDecl) {
+	g.ensure_extern_sig_type_decl(node.return_type)
+	for param in node.params {
+		g.ensure_extern_sig_type_decl(param.typ)
+	}
+}
+
+fn (mut g Gen) ensure_extern_sig_type_decl(typ_ ast.Type) {
+	if typ_ == 0 {
+		return
+	}
+	typ := typ_.clear_option_and_result().set_nr_muls(0).clear_flags(.generic, .variadic)
+	if typ == 0 {
+		return
+	}
+	sym := g.table.sym(typ)
+	if sym.is_builtin || sym.name in ['byte', 'i32', 'C.FILE'] {
+		return
+	}
+	if sym.idx in g.table.used_features.used_syms {
+		return
+	}
+	if sym.info is ast.Struct {
+		if sym.language == .c && sym.cname.starts_with('C__') && !sym.info.is_anon {
+			c_struct_name := sym.cname[3..]
+			if c_struct_name != 'va_list' {
+				if sym.info.is_typedef {
+					g.typedefs.writeln('typedef struct ${c_struct_name} ${c_struct_name};')
+				} else {
+					g.typedefs.writeln('struct ${c_struct_name};')
+				}
+			}
+		} else {
+			g.typedefs.writeln('typedef struct ${sym.cname} ${sym.cname};')
+		}
+	}
+}
+
 fn (mut g Gen) is_used_by_main(node ast.FnDecl) bool {
 	$if trace_unused_by_main ? {
 		defer(fn) {
@@ -837,6 +875,9 @@ fn (mut g Gen) fn_decl(node ast.FnDecl) {
 	}
 	if node.language == .c && (node.is_c_extern || g.should_emit_c_extern_decl(node)) {
 		g.inside_c_extern = true
+		if g.pref.skip_unused {
+			g.ensure_extern_sig_type_decls(node)
+		}
 	}
 
 	g.gen_attrs(node.attrs)