Add missing ExternalFile tags to object_dmask and object_osn

[Thar0]

Skeletons in these objects have references to display lists in gameplay_keep, so they should rely on gameplay_keep as an ExternalFile.

Unfortunately though, adding these to the xmls do not appear to actually replace the hardcoded gameplay_keep pointers with symbols.. a ZAPD bug?

[AngheloAlf]

I don't think this PR actually does anything at all since we list gameplay_keep as a dependency for every other object by listing it on the global config (https://github.com/zeldaret/mm/blob/main/tools/ZAPDConfigs/MM/Config.xml#L7). This needs to be fixed on ZAPD's side

[Thar0]

Lots of other objects explicitly list gameplay_keep as an ExternalFile, so if nothing else it makes these objects consistent with the rest. But yes, a ZAPD fix is needed.

[AngheloAlf]

In that case I think we should probably clean up the xmls. I guess we could remove gameplay_keep from the global config and just be explicit about it. Either way, that decision doesn't affect this PR.