cgen: fix missing defer in fn main()

vlib/v/gen/cgen.v

@@ -885,11 +885,13 @@ fn (g mut Gen) gen_fn_decl(it ast.FnDecl) {
 		if g.is_test {
 			verror('test files cannot have function `main`')
 		}
-		g.writeln('\treturn 0;')
 	}
 	if g.defer_stmts.len > 0 {
 		g.write_defer_stmts()
 	}
+	if is_main {
+		g.writeln('\treturn 0;')
+	}
 	g.writeln('}')
 	g.defer_stmts = []
 	g.fn_decl = 0