Fix parsing macros with only a variadic argument

Function-like macros with only a variadic argument (e.g. MACRO(...))
were not parsed correctly, so linking to them would fail.

Change-Id: I1126f76473d82b717629f70d0d82245ce46dae86
Reviewed-on: https://pigweed-review.googlesource.com/c/third_party/github/sphinx-contrib/doxylink/+/386618