[valhalla] gitmodules gone in valhalla

[farfromrefug]

@mtehver there must have been some issue there https://github.com/nutiteq/valhalla/tree/mbtiles-support because the .gitmodules is gone. Preventing the cloning of it.
I manually added it back here Akylas/valhalla@d682c03 to fix it

[mtehver]

I think I removed it as some submodules of Valhalla failed to update and they were not really needed (libs-external already contains a copy of rapidjson and date, for example).