net: qrtr: combine nameservice into main module

Previously with CONFIG_QRTR=m a separate ns.ko would be built which
wasn't done on purpose and should be included in qrtr.ko.

Rename qrtr.c to af_qrtr.c so we can build a qrtr.ko with both af_qrtr.c
and ns.c.

Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Tested-By: Steev Klimaszewski <steev@kali.org>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://lore.kernel.org/r/20210928171156.6353-1-luca@z3ntu.xyz
Signed-off-by: Jakub Kicinski <kuba@kernel.org>