portaudio: disable parallel build

The portaudio build system contains a race condition, which gets triggered
by high BR2_JLEVEL settings - It tries to use libportaudio.la before it is
created:

make[3]: *** No rule to make target `../../../lib/libportaudio.la',
needed by `libportaudiocpp.la'.  Stop.

Fix it by disabling parallel build.

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