mk: use make silent flag to print HTML doc version

mk: use make silent flag to print HTML doc version

Depending on the environment, make might echo the command being ran.
In mk/rte.sdkdoc.mk make is used to print the DPDK version to be
piped to doxygen. This causes the following to be written:

<div id="projectname">DPDK
&#160;<span id="projectnumber">/usr/bin/make-f/build/dpdk-jYjqnr/
 dpdk-16.11.2/mk/rte.sdkconfig.mkshowversion</span>
</div>

Use -s (--silent) to prevent echoing.

Signed-off-by: Luca Boccassi <luca.boccassi@gmail.com>
Acked-by: John McNamara <john.mcnamara@intel.com>