rt-tests: fix redo install error

rt-tests: fix redo install error

Currently gzip on host is used. If host gzip is provided by pigz, it
fails to redo install that pigz can't handle the option after file name.
When run command for target install in Makefile:

gzip src/backfire/backfire.4 -c > OUTPUT_FILE

File src/backfire/backfire.4 is zipped into backfire.4.gz but the
OUTPUT_FILE is empty. When rerun do_install, it shows warning:

| gzip: src/backfire/backfire.4 does not exist -- skipping

and empty manual gzip files are created:

$ file image/usr/share/man/man4/backfire.4.gz
image/usr/share/man/man4/backfire.4.gz: empty

Fix it by putting option '-c' before the file name.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>