devel/doxygen: Fix build with ninja 1.13.1

Add missing dependencies in examples. Upstream added baseexample.cfg in
[1], but example_test.cpp and include_test.cpp race to be copied into
BUILD_WRKSRC. This error manifests itself especially when building using
a single job as is the default case in poudriere.

[1] https://github.com/doxygen/doxygen/commit/ceed504cb5e64a1b8a04174489642391a02ed7e3

PR: 279187
Approved by: portmgr (blanket)