Fix LMS zeroization using wrong sizeof type

Causing a buffer write out of bounds

Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/library/lms.c b/library/lms.c
index 7ec2b10..653530c 100644
--- a/library/lms.c
+++ b/library/lms.c
@@ -525,7 +525,7 @@
         mbedtls_free( ctx->ots_public_keys );
     }
 
-    mbedtls_platform_zeroize( ctx, sizeof( mbedtls_lms_public_t ) );
+    mbedtls_platform_zeroize( ctx, sizeof( mbedtls_lms_private_t ) );
 }