Avoid bison-isms when using yacc.

Avoid bison-isms when using yacc.

YYPRINT is a bison-ism so c_print_token() ends up being unused when yacc is
used which makes gcc unhappy.  Make sure we only define YYPRINT and
c_print_token() when bison is used to generate the parser.

gdb/ChangeLog:

        * c-exp.y (YYPRINT, c_print_token): Only define if YYBISON is
        defined.