ALGORITHMS: Fix compilation on Linux

Wrapping all KissFFT headers in their own namespace lead to an issue:
these headers included standard C header which then where locked
in a namespace instead of being in an anonymous one, preventing
compilation.

Signed-off-by: G4m4 <misept.dieseneuf@gmail.com>
MISC: Minor style fixes

- signed/unsigned mismatches
- extra ";"

Signed-off-by: G4m4 <misept.dieseneuf@gmail.com>