Always add -DPIC to YASM flags when pic is enabled.

This covers the previous case of x86_64 and shared, but also
works with --enable-pic.
For 32 bit x86 it currently makes no difference since x86inc.asm
ignores -DPIC for non-x86_64.

Originally committed as revision 20476 to svn://svn.ffmpeg.org/ffmpeg/trunk