webkit: fix staging install

Parallel 'make install' seems to deadlock here (make 3.81). Work around it
by disabling parallel make for the install step.

We only disable parallel make for the install step, as the build step works
with parallel make, and webkit is a huge package, so build time goes up
significantly with make -j1.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>