kmod: really pass --disable-manpages

The line below overwrites KMOD_CONF_OPT, so move the --disable-manpages
options below.

Fixes http://autobuild.buildroot.net/results/842/842131307c171e1bc5a1e918996d53b00ddb1be3/



Signed-off-by: Peter Korsgaard <peter@korsgaard.com>