Add ff_celp_circ_addf() function to be used for sparse vector circular

Add ff_celp_circ_addf() function to be used for sparse vector circular
convolution in the upcoming AMR-NB floating point decoder. The function scales
and adds a vector, that is lagged by some offset, to another vector with the
same number of elements.

Patch by Colin McQuillan ( m.niloc googlemail com )

Originally committed as revision 19634 to svn://svn.ffmpeg.org/ffmpeg/trunk