Expose QCELP's floating-point LSP-to-LPC function

Expose QCELP's floating-point LSP-to-LPC function
qcelp_lsp exported a single function, ff_acelp_lspd2lpc, which was not
specific to qcelp. It can be kept with its fixed-point version
ff_acelp_lsp2lpc in lpc.c.

Patch by Colin McQuillan ( m.niloc googlemail com )

Originally committed as revision 19571 to svn://svn.ffmpeg.org/ffmpeg/trunk