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/library/rsa.c b/library/rsa.c
index b6fea49..557faaf 100644
--- a/library/rsa.c
+++ b/library/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"
@@ -1229,16 +1230,6 @@
 }
 
 /*
- * 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,