Use styled_string for "show logging filename"

Use styled_string for "show logging filename"

This changes "show logging filename" to style its output.

gdb/ChangeLog
2019-10-01  Tom Tromey  <tom@tromey.com>

	* cli/cli-logging.c (show_logging_filename): Use styled_string.

gdb/testsuite/ChangeLog
2019-10-01  Tom Tromey  <tom@tromey.com>

	* gdb.base/style.exp: Test "show logging filename".