fixed-point: changed find_spectral_pitch() to use single-precision (16-bit) FFT.

This involved adding kfft_single.[ch] that redefines kiss_fft a second time
with a different prefix. All this is still a bit of a mess now. The mask
had to be converted to 16-bit input, but we're still using floats to apply it.

libcelt/kfft_single.c

@@ -0,0 +1,13 @@
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
+
+#ifdef FIXED_POINT
+
+#include "kfft_single.h"
+
+#define SKIP_CONFIG_H
+#include "kiss_fft.c"
+#include "kiss_fftr.c"
+
+#endif