Fix requires expression bug

Author: KangarooKoala

cxxheaderparser/parser.py

@@ -836,7 +836,7 @@ def _parse_requires(
                 elif tok.type == "requires":
                     rawtoks.append(tok)
                     for tt in ("(", "{"):
-                        tok = self._next_token_must_be("{")
+                        tok = self._next_token_must_be(tt)
                         rawtoks.extend(self._consume_balanced_tokens(tok))
                 else:
                     tok = self._parse_requires_segment(tok, rawtoks)