CMSIS-DSP: Added missing "const"

Added apparently missing "const" to ref_biquad_cascade_df2T_f64(), which
was causing compile errors with IAR toolchain.

Signed-off-by: TTornblom <thomas.tornblom@iar.com>

DSP_Lib_TestSuite/RefLibs/src/FilteringFunctions/biquad.c

@@ -158,7 +158,7 @@ void ref_biquad_cascade_df2T_f64(
         float64_t *pIn = pSrc;                         /*  source pointer            */
         float64_t *pOut = pDst;                        /*  destination pointer       */
         float64_t *pState = S->pState;                 /*  State pointer             */
-   float64_t *pCoeffs = S->pCoeffs;               /*  coefficient pointer       */
+  const float64_t *pCoeffs = S->pCoeffs;               /*  coefficient pointer       */
         float64_t acc;                                 /*  accumulator               */
         float64_t b0, b1, b2, a1, a2;                  /*  Filter coefficients       */
         float64_t Xn;                                  /*  temporary input           */