Use unique_ptr instead of calling free directly

Seems kind of silly, but sure, whatever you say clang-tidy.

Change-Id: Ieefaf3ee9d18eeee0d02ae8b2cd2873b52741cb6
diff --git a/src/yang_schema.cpp b/src/yang_schema.cpp
index 575e241..711cdbd 100644
--- a/src/yang_schema.cpp
+++ b/src/yang_schema.cpp
@@ -398,7 +398,7 @@
     };
 
     auto deleter = [](void* data) {
-        free(data);
+        free(data); // NOLINT(cppcoreguidelines-owning-memory,cppcoreguidelines-no-malloc)
     };
     m_context->add_missing_module_callback(lambda, deleter);
 }