[WIP/FIX] Lock write operations to XMPP

I *think* that fixes stream errors with two threads writing at once.
Even if it doesn't, I think it's always a nice-to-have.

src/include/XMPP.h

@@ -3,9 +3,14 @@
 
 #include <Cytoplasm/Stream.h>
 
+#include <pthread.h>
+
 #include <XML.h>
 
 typedef struct XMPPComponent {
+    /* A lock for all write operations */
+    pthread_mutex_t write_lock;
+
     char *host;
     Stream *stream;
 } XMPPComponent;