bignum_mod: Added a typedef for OPT_RED function pointer.

Signed-off-by: Minos Galanakis <minos.galanakis@arm.com>
diff --git a/library/ecp_curves.c b/library/ecp_curves.c
index cb94196..e161fd4 100644
--- a/library/ecp_curves.c
+++ b/library/ecp_curves.c
@@ -5833,7 +5833,7 @@
                               const mbedtls_ecp_group_id id,
                               const mbedtls_ecp_modulus_type ctype)
 {
-    int (*modp)(mbedtls_mpi_uint *X, size_t X_limbs) = NULL;
+    mbedtls_mpi_modp_fn modp = NULL;
     mbedtls_mpi_uint *p = NULL;
     size_t p_limbs;