m2mbase: fix compiler warning on 64bit arch
m2mbase: fix compiler warning on 64bit arch

Use standard printf format specifier for printing a pointer value.

Fix for this warning:
--8<---8<--
warning: format ‘%x’ expects argument of type ‘unsigned int’,
but argument 4 has type ‘M2MObservationHandler*’ [-Wformat=]

tridonic-com/mbed-client,ARMmbed/mbed-client,tridonic-com/mbed-client,ARMmbed/mbed-client,tridonic-com/mbed-client,ARMmbed/mbed-client