Fix filename extension of common include file for gruvbox-* themes

[SpotlightKid]

The common include file for the gruvbox-* was not installed, because it has an .inc extension instead of .rasi. Also, when including files, rofi always adds the .rasi extension, so I renamed the .inc file and fixed the includes in the themes using it.

BTW, in the rofi distribution this is already fixed in the same way.

[DaveDavenport]

I think the 'inc' might be better, otherwise common file shows up in theme selector.. (I should fix this in rofi).

[SpotlightKid]

Yes, that's what happens with the globally installed gruvbox-common.rasi file from the rofi distribution.

But, as it is now, the gruvbox themes installed from this repo do not work at all, because the .inc doesn't get installed, and even if it is, it will not be found.

BTW, I think rofi-theme-selector should filter out themes in the system themes directory, which are overwritten by a user theme with the same file name.