fix ssr_gc_function signature mismatch in ssr gain control

[netliomax25-code]

1. ssr_gc_function is declared in ssr.h with five parameters but defined in ssr.c with a sixth band parameter, and the two calls in ssr_gain_control() disagree: the long-window path passes six arguments while the eight-short path passes five.
2. with SSR_DEC enabled this does not compile (conflicting types for ssr_gc_function, and too many arguments at the long-window call), and band is never read in the function body.
3. ssr_decode() also calls ssr_ipqf() without including ssr_ipqf.h, a second compile error on the same path.

Removed the unused band parameter so the definition and both calls match the declaration, and added the missing ssr_ipqf.h include. ssr.c is compiled only under SSR_DEC, so the default build is unchanged.