Use 'frame' instead of 'signal' in the silk code.

On MacOS, stdlib.h ends up including sys/signal.h, generating
warnings about the local variables called 'signal' shadowing
the global symbol signal(3). Tested with XCode 4.1 on
MacOS X 10.7.0.

The signal buffers passed in are generally frames being processed,
and the code already uses the term frame and frame_length elsewhere,
so I've resolved the warning by renaming signal and signal_* locals
and parameters to frame and frame_*.