Fix gdb/remote.c build failure

Add const qualifier to fix

/S/gdb/common/gdb_locale.h:35:27: error: deprecated conversion from string constant to 'char*' [-Werror=write-strings]
 # define _(String) (String)
                           ^
/S/gdb/remote.c:12844:19: note: in expansion of macro '_'
   char *err_msg = _("Tracepoint packet too large for target.");
                   ^
gdb/ChangeLog:

	* remote.c (remote_target::download_tracepoint): Change char* to
	const char*.