cifs-utils: disable man page generation

cifs-utils: disable man page generation

Buildroot does not generate documentation for target.

This fixes the build on hosts where the rst2man command does not support
the --syntax-highlight parameter.

Fixes:
http://autobuild.buildroot.net/results/265/2655c0e1fa3ad0a10b4aed39a17feead94e47bfb/
http://autobuild.buildroot.net/results/92d/92d7c608f717bbfe01ecfb9bc9604cb303d8594c/
http://autobuild.buildroot.net/results/4b9/4b95404a89a595ca9c1e3df912169e9d36ff2bd7/

Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>