modemmanager: change permissions on dbus file

modemmanager: change permissions on dbus file

/etc/dbus-1/system.d/org.freedesktop.ModemManager1.conf needs to be 644
so that it can be read to prevent the following since dbus runs as an
unprivileged user:

dbus-daemon[12465]: Encountered error 'Failed to open "/etc/dbus-1/system.d/org.freedesktop.ModemManager1.conf": Permission denied' while parsing '/etc/dbus-1/system.d/org.freedesktop.ModemManager1.conf

Build system: x86/64
Build-tested: x86/64-glibc
Run-tested: x86/64-glibc

Signed-off-by: John Audia <[email protected]>