rubyspec compiles the extension libraries for capi specs in a temporary
directory, and after that, deletes the directory.  gcov resurrects the
directory to save coverage statistics (.gcda files).  However, the
source and profiling artifacts (.gcno files) have been deleted.
So, we need to ignore the .gcda files.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59260 b2dd03c8-39d4-4d8f-98ff-823fe69b080e