ecc.c: add (void)rng to wc_ecc_mulmod_ex2() to cover WOLFSSL_SP_MATH case.

wolfcrypt/src/ecc.c

@@ -10122,6 +10122,8 @@ int wc_ecc_mulmod_ex2(mp_int* k, ecc_point *G, ecc_point *R, mp_int* a,
 
     return err;
 #else
+    (void)rng;
+
    if (k == NULL || G == NULL || R == NULL || a == NULL || modulus == NULL ||
                                                                 order == NULL) {
         return ECC_BAD_ARG_E;