Handle kprobe struct field symbol_name as either char * or const char *.

Handle kprobe struct field symbol_name as either char * or const char *.

Older (2.6.18) based kernels defined struct kprobes symbol_name as char *,
but newer (2.6.27) based kernels defined the same field as const char *.

* tapsets.cxx (kprobe_derived_probe_group::emit_module_init):
  Always cast to char * when assigning to kprobes.symbol_name field.