meson: handle missing xkbcomp.pc better
Applying get_pkgconfig_variable() to a not-found dependency was always documented as an error, but meson 0.49 now actually raises an error[1]:
meson.build:110:4: ERROR: 'xkbcomp' is not a pkgconfig dependency
Check xkbcomp_dep is a suitable dependency type before applying get_pkgconfig_variable() to it.
[1] but this is more by accident than design (see the discusssion at [2] et seq.), so who knows where things will come to rest...
[2] https://github.com/mesonbuild/meson/pull/4444#issuecomment-442443301