Add MBEDTLS_LMS_PRIVATE define

To enable private key operations

Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/library/lmots.c b/library/lmots.c
index d92d385..bf66449 100644
--- a/library/lmots.c
+++ b/library/lmots.c
@@ -455,6 +455,8 @@
     return( 0 );
 }
 
+#ifdef MBEDTLS_LMS_PRIVATE
+
 void mbedtls_lmots_init_private( mbedtls_lmots_private_t *ctx )
 {
     mbedtls_platform_zeroize( ctx, sizeof( mbedtls_lmots_private_t  ) ) ;
@@ -716,4 +718,5 @@
     return( 0 );
 }
 
+#endif /* MBEDTLS_LMS_PRIVATE */
 #endif /* MBEDTLS_LMS_C */