fix multithreaded make

this is supposed to fix a race in our Makefile, which can result in Errors like:
CPP     xbmc/utils/XBMCTinyXML.o
AR      xbmc/pvr/windows/pvrwindows.a
make[2]: Leaving directory `/build/buildd/xbmc-13.0~git20140203.0211-848b0c3/xbmc/pvr/windows'
CPP     xbmc/utils/XMLUtils.o
make[1]: *** No rule to make target `lib/ffmpeg/libavcodec/libavcodec.a', needed by `xbmc.bin'.  Stop.
make[1]: *** Waiting for unfinished jobs....

@t-nelson: ping