Added regex-compat to dependencies

Build fails with message:
...
Configuring xmobar-0.19...
setup: At least the following dependencies are missing:
regex-compat -any

xmobar itroduced regex-compat dependency with commit https://github.com/jaor/xmobar/commit/85fdac20a36675559802dd215a81f640b91693fe
Merge pull request #289 from akolesen/master

Added regex-compat to dependencies