xmlschemastypes: ensure thread safe global initialization Relying on a plain integer flag, with no synchronization primitives does not give thread-safe initialization. All reads & writes of the xmlSchemaTypesInitialized flag need to be protected by a mutex to ensure suitable memory barriers & thus correct ordering wrt any speculative execution. A separate internal initializer tied to xmlParserInit is used to create the mutex used for synchronization, similarly to how catalog.c works. Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>