Readd pthread_mutex.h include on ngx_fetch.cc.

Messed up the merge of 0290f52a8892c87eed4103e1990a07d0838fd05e originally, add the include back.
diff --git a/src/ngx_fetch.cc b/src/ngx_fetch.cc
index d57811e..299ba2e 100644
--- a/src/ngx_fetch.cc
+++ b/src/ngx_fetch.cc
@@ -56,6 +56,7 @@
 #include "net/instaweb/util/public/message_handler.h"
 #include "net/instaweb/util/public/pool.h"
 #include "net/instaweb/util/public/pool_element.h"
+#include "net/instaweb/util/public/pthread_mutex.h"
 #include "net/instaweb/util/public/statistics.h"
 #include "net/instaweb/util/public/string_writer.h"
 #include "net/instaweb/util/public/string_util.h"