gthumb: Disable parallel make for gthumb install

gthumb: Disable parallel make for gthumb install

With PARALLEL_MAKE set to 14, I frequently see the gthumb do_install
task hang. Make is spinning at 100% CPU and the build makes no
more progress.

The following work-around proposed by Richard Purdie allows progress
to be made.

(From OE-Core rev: 01e14ae25c133e46603d01a72d0372c111e12f11)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
CC: Joshua Lock <josh@linux.intel.com>
CC: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>