musepack: disable parallel build

The musepack build system contains a race condition, which gets triggered by
high BR2_JLEVEL settings - It tries to link against libmpcdec before it is
created:

Linking C executable mpcgain
/scratch/peko/host/usr/lib/gcc/arm-unknown-linux-uclibcgnueabi/4.4.6/../../../../arm-unknown-linux-uclibcgnueabi/bin/ld: cannot find -lmpcdec
collect2: ld returned 1 exit status
make[3]: *** [mpcgain/mpcgain] Error 1

Fix it by disabling parallel build.

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