Enable qcom-soc plugin by default

[Shawn Guo]

Hi Aleksander,

I'm running ModemManager 1.18 on Lenovo Yoga C630 laptop which is
powered by Qualcomm SDM850 SoC.  It works quite out of box for me with
the only stumble on that qcom-soc plugin is not enabled by default.
It seems that you made it so on purpose with commit[1].  As people are
running distros like Debian, Archlinux, Fedora, etc. on such laptops
powered by Qualcomm Snapdragon SoCs, I'm wondering if we can enable
qcom-soc plugin by default to make it easier for distros to build the
MM aarch64 packages.  Thanks! Regards

Shawn

[1] https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/commit/65560dd8854f01eec1b28587c37d544bfff360d3