Change the fix_quasi_reduction function to static

Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>

library/bignum_mod_raw_invasive.h

@@ -28,6 +28,8 @@
 #include "mbedtls/bignum.h"
 #include "bignum_mod.h"
 
+#if defined(MBEDTLS_TEST_HOOKS)
+
 /** Convert an MPI to its canonical representative.
  *
  * \note Currently handles the case when `N->int_rep` is
@@ -44,4 +46,6 @@ MBEDTLS_STATIC_TESTABLE
 int mbedtls_mpi_mod_raw_fix_quasi_reduction(mbedtls_mpi_uint *X,
                                             const mbedtls_mpi_mod_modulus *N);
 
+#endif /* MBEDTLS_TEST_HOOKS */
+
 #endif /* MBEDTLS_BIGNUM_MOD_RAW_INVASIVE_H */