tags: Fix erroneous pattern match in a comment

Apparently, ctags applies the rules before deleting comments:

ctags: Warning: include/linux/completion.h:22: null expansion of name pattern "\2"

Work around this particular case by requiring the group to contain at
least one character. Leave the other patters as they are, until a better
solution is found.

Signed-off-by: Michal Marek <mmarek@suse.com>