Add a missing bracket in ifdef for __cplusplus

diff --git a/include/mbedtls/rsa_internal.h b/include/mbedtls/rsa_internal.h
index bcb3c94..12e0f6b 100644
--- a/include/mbedtls/rsa_internal.h
+++ b/include/mbedtls/rsa_internal.h
@@ -213,4 +213,8 @@
                               const mbedtls_mpi *D,  const mbedtls_mpi *DP,
                               const mbedtls_mpi *DQ, const mbedtls_mpi *QP );
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* rsa_internal.h */