This fix renames the `gruvbox-common.inc` to `gruvbox-common.inc.rasi`,
which leads to the filename being installed by `install.sh` and found by
Rofi. `gruvbox-common.inc.rasi` is the file name Rofi expects to find,
when I start Rofi with the gruvbox dark theme.
Signed-off-by: Patrick Seidensal <patrick@seidensal.info>