Put double-quotes around the install path in the "install-headers" section

of the makefiles for libavcodec, libavformat and libavutil.
Fixes installing into paths with spaces in them, i.e. Windows.
patch by Fredrik Orderud < fredrik . orderud -- at -- idi . ntnu . no >

Originally committed as revision 4680 to svn://svn.ffmpeg.org/ffmpeg/trunk