libkms: add libdrm to Requires.private

Analogous to last two changes (amdgpu and radeon).

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Jakob Bornecrantz <wallbraker@gmail.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>