[Buildroot] modem-manager: build issue

[Yegor Yefremov]

Current mm 1.6.12 produces following errors both when compiling an ARM
or MIPS target:

  CC       ModemManager-mm-base-modem.o
mm-base-manager.c: In function ‘handle_set_logging’:
mm-base-manager.c:680:15: error: assignment from incompatible pointer
type [-Werror=incompatible-pointer-types]
     ctx->self = g_object_ref (manager);
               ^
mm-base-manager.c: In function ‘handle_scan_devices’:
mm-base-manager.c:736:15: error: assignment from incompatible pointer
type [-Werror=incompatible-pointer-types]
     ctx->self = g_object_ref (manager);
               ^
mm-device.c: In function ‘mm_device_get_plugin’:
mm-device.c:628:12: error: return from incompatible pointer type
[-Werror=incompatible-pointer-types]
     return (self->priv->plugin ?

So far I've seen only one build failure in our autobuilders [0]. I can
reproduce this error building mm 1.6.12 locally with my host GCC. But
the master branch of mm compiles without any issue. We don't seem to
be the only ones who is affected [1], [2].

Any ideas?

[0] http://autobuild.buildroot.net/results/5d9e299ff12726d07e8a584a213c1d2a2e419594/
[1] https://fedorapeople.org/groups/risc-v/logs/ModemManager-1.6.12-3.fc28.src.rpm/build.log
[2] https://www.linuxquestions.org/questions/slackware-14/slackware-from-scratch-and-x11-4175560702/page19.html

Yegor