Use of 'class' as a field name in Kannel

[Paul Keogh]

The latest MSG(sms... definition in the msg-decl.h file
includes a field name of 'class'. This makes it very 
difficult to use in C++ srcs. Kannel has and probably
never will any C++ src but other applications written
in C++ may wish to use the Msg struct and/or some
of the Msg functions.

Can we change it to 'sms_class' ?