[Pal/Linux-SGX] pal-sgx-sign: Always output sgx.allow_file_creation

If user didn't specify `sgx.allow_file_creation` in manifest template, it is
explicitly added to the generated manifest as `sgx.allow_file_creation = 0`.

Pal/src/host/Linux-SGX/signer/pal-sgx-sign

@@ -850,6 +850,9 @@ def main_sign(args):
             enclave_heap_min = 0
             manifest['sgx.static_address'] = '0'
 
+    if manifest.get('sgx.allow_file_creation', None) is None:
+        manifest['sgx.allow_file_creation'] = '0'
+
     output_manifest(args['output'], manifest, manifest_layout)
 
     memory_areas = [