Cast result of obstack_next_free

Cast result of obstack_next_free

obstack_next_free is supposed to return a void*, rather than a char*
as it does currently.  Avoid warning on void* arithmetic when
obstack_next_free gets it proper return type.

	* cp-valprint.c (cp_print_value_fields): Cast obstack_next_free
	to char* before doing pointer arithmetic.