parser: fix embed file to_string() in if guard (#16334)

vlib/v/embed_file/tests/a.txt

@@ -0,0 +1 @@
+test

vlib/v/embed_file/tests/embed_file_to_string_test.v

@@ -0,0 +1,9 @@
+fn test_embed_file_to_string() {
+	s := if true {
+		$embed_file('./a.txt').to_string()
+	} else {
+		''
+	}
+	println(s)
+	assert s.trim_space() == 'test'
+}

vlib/v/parser/parser.v

@@ -1002,7 +1002,7 @@ pub fn (mut p Parser) stmt(is_top_level bool) ast.Stmt {
 				}
 				.name {
 					mut pos := p.tok.pos()
-					expr := p.comptime_call()
+					expr := p.expr(0)
 					pos.update_last_line(p.prev_tok.line_nr)
 					return ast.ExprStmt{
 						expr: expr