avfilter/window_func: use a constant instead of acosh()

Should fix compilation with non C99 compilers like msvc 2012, where
acosh() is not available.
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavfilter/window_func.c

@@ -117,7 +117,7 @@ void ff_generate_window_func(float *lut, int N, int win_func, float *overlap)
         *overlap = 0.33;
         break;
     case WFUNC_DOLPH: {
-        double b = cosh(acosh(pow(10., 3)) / (N-1)), sum, t, c, norm = 0;
+        double b = cosh(7.6009022095419887 / (N-1)), sum, t, c, norm = 0;
         int j;
         for (c = 1 - 1 / (b*b), n = (N-1) / 2; n >= 0; --n) {
             for (sum = !n, b = t = j = 1; j <= n && sum != t; b *= (n-j) * (1./j), ++j)