Adds 3rd clause to CELT license

celt/_kiss_fft_guts.h

celt/arch.h

celt/bands.c

celt/bands.h

celt/celt.c

celt/celt.h

celt/celt_lpc.c

celt/celt_lpc.h

celt/cwrs.c

celt/cwrs.h

celt/dump_modes/dump_modes.c

celt/ecintrin.h

celt/entcode.c

celt/entcode.h

celt/entdec.c

celt/entdec.h

celt/entenc.c

celt/entenc.h

celt/fixed_c5x.h

celt/fixed_c6x.h