Add header for mbedtls_mpi_exp_mod_unsafe()

To silence no previous prototype warnings. And this is the proper way to
do it anyway.

Signed-off-by: Janos Follath <janos.follath@arm.com>
diff --git a/tf-psa-crypto/drivers/builtin/src/bignum.c b/tf-psa-crypto/drivers/builtin/src/bignum.c
index b1f9c1b..4244909 100644
--- a/tf-psa-crypto/drivers/builtin/src/bignum.c
+++ b/tf-psa-crypto/drivers/builtin/src/bignum.c
@@ -27,6 +27,7 @@
 
 #include "mbedtls/bignum.h"
 #include "bignum_core.h"
+#include "bignum_internal.h"
 #include "bn_mul.h"
 #include "mbedtls/platform_util.h"
 #include "mbedtls/error.h"
diff --git a/tf-psa-crypto/drivers/builtin/src/rsa.c b/tf-psa-crypto/drivers/builtin/src/rsa.c
index 3df00c0..16912fd 100644
--- a/tf-psa-crypto/drivers/builtin/src/rsa.c
+++ b/tf-psa-crypto/drivers/builtin/src/rsa.c
@@ -29,6 +29,7 @@
 
 #include "mbedtls/rsa.h"
 #include "bignum_core.h"
+#include "bignum_internal.h"
 #include "rsa_alt_helpers.h"
 #include "rsa_internal.h"
 #include "mbedtls/oid.h"
@@ -1227,16 +1228,6 @@
 }
 
 /*
- * This function is identical to mbedtls_mpi_exp_mod() the only difference is that this function is
- * not constant time.
- *
- * WARNING! This function is not constant time.
- */
-int mbedtls_mpi_exp_mod_unsafe(mbedtls_mpi *X, const mbedtls_mpi *A,
-                               const mbedtls_mpi *E, const mbedtls_mpi *N,
-                               mbedtls_mpi *prec_RR);
-
-/*
  * Do an RSA public key operation
  */
 int mbedtls_rsa_public(mbedtls_rsa_context *ctx,
diff --git a/tf-psa-crypto/tests/suites/test_suite_bignum.function b/tf-psa-crypto/tests/suites/test_suite_bignum.function
index 1830e5a..3ac4e10 100644
--- a/tf-psa-crypto/tests/suites/test_suite_bignum.function
+++ b/tf-psa-crypto/tests/suites/test_suite_bignum.function
@@ -3,6 +3,7 @@
 #include "mbedtls/entropy.h"
 #include "constant_time_internal.h"
 #include "bignum_core.h"
+#include "bignum_internal.h"
 #include "test/constant_flow.h"
 
 #if MBEDTLS_MPI_MAX_BITS > 792