iface-modem: actually respect build options

iface-modem does not include config.h so it doesnt actually have the meson generated build defines to check against.

I found this while trying to use AT commands over D-Bus by enabling at_command_via_dbus, confirming that Meson sees it, defines it in config.h but then I still got the: "Cannot send AT command to modem: operation only allowed in debug mode".

Signed-off-by: Robert Marko robert.marko@sartura.hr