-Wwrite-strings: Constify warning_pre_print

-Wwrite-strings: Constify warning_pre_print

-Wwrite-strings flags a warning here:

  char *warning_pre_print = "\nwarning: ";

gdb/ChangeLog:
2017-04-05  Pedro Alves	 <palves@redhat.com>

	* main.c (captured_main_1): Use gdb::unique_xmalloc_ptr to manage
	the memory of the temporary warning_pre_print override.
	* utils.c (warning_pre_print): Constify.
	* utils.h (warning_pre_print): Constify.