Only enable fix_quasi_reduction when testing

Avoid compiler error due to the fix_quasi_reduction function
is static and has not been used.

Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
diff --git a/library/bignum_mod_raw.c b/library/bignum_mod_raw.c
index 049cdb2..6ac9e72 100644
--- a/library/bignum_mod_raw.c
+++ b/library/bignum_mod_raw.c
@@ -118,15 +118,7 @@
     (void) mbedtls_mpi_core_add_if(X, N->p, N->limbs, (unsigned) c);
 }
 
-void mbedtls_mpi_mod_raw_mul(mbedtls_mpi_uint *X,
-                             const mbedtls_mpi_uint *A,
-                             const mbedtls_mpi_uint *B,
-                             const mbedtls_mpi_mod_modulus *N,
-                             mbedtls_mpi_uint *T)
-{
-    mbedtls_mpi_core_montmul(X, A, B, N->limbs, N->p, N->limbs,
-                             N->rep.mont.mm, T);
-}
+#if defined(MBEDTLS_TEST_HOOKS)
 
 MBEDTLS_STATIC_TESTABLE
 void mbedtls_mpi_mod_raw_fix_quasi_reduction(mbedtls_mpi_uint *X,
@@ -137,6 +129,18 @@
     (void) mbedtls_mpi_core_add_if(X, N->p, N->limbs, (unsigned) c);
 }
 
+#endif /* MBEDTLS_TEST_HOOKS */
+
+void mbedtls_mpi_mod_raw_mul(mbedtls_mpi_uint *X,
+                             const mbedtls_mpi_uint *A,
+                             const mbedtls_mpi_uint *B,
+                             const mbedtls_mpi_mod_modulus *N,
+                             mbedtls_mpi_uint *T)
+{
+    mbedtls_mpi_core_montmul(X, A, B, N->limbs, N->p, N->limbs,
+                             N->rep.mont.mm, T);
+}
+
 /* END MERGE SLOT 2 */
 
 /* BEGIN MERGE SLOT 3 */