silk: move down- and upsamplers constants to the header file.

These are only being used outside if the translation unit and only in
a few codepaths; by making them static in the header, the compiler can
optimize them quickly.

This avoids GOT-based access to the external symbol on Unix, and can
allow for constant propagation to optimize the code.

Signed-off-by: Diego Elio Pettenò <flameeyes@flameeyes.eu>