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"