scripts/tags.sh: add regular expression replacement pattern for memcg

Currently, while using ctags to read code, we would get stumbled on
PageCgroup* symbols: no definition found. And it is quite dull to
manually dig it out.

This patch adds regular expression replacement pattern for such symbols,
like what have done for the PageXXX flag. It will teach ctags to find
out the definition for us.

Signed-off-by: Jianyu Zhan <nasa4836@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>