tekui: disable parallel build

tekui: disable parallel build

It seems tekui has parallel build issues since November 2017:
 - Fatal error: can't create build/posix/directfb_lua.lo: No such file or directory
 - /home/peko/autobuild/instance-3/output/host/bin/microblazeel-buildroot-linux-uclibc-ar: ../../lib/posix/libtekdebug.a: No such file or directory
 - Fatal error: can't create build/posix/visual_mod.lo: No such file or directory

So disable parallel build

Fixes:
 - http://autobuild.buildroot.org/results/0732568fcbaa6829154fa91c352b52f074384df0
 - http://autobuild.buildroot.org/results/580593e79bc4ecdea1dc71d16607e5c88f87403c

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>