gn-build: Get //stack:stack to compile

This CL adds a missing include to get the //stack:stack target to compile.

BUG=21339022

Change-Id: Ie41816514c97ddcb1e4f1f0b9481364a563d8b9b
diff --git a/stack/btu/btu_task.c b/stack/btu/btu_task.c
index e2b899e..41170f0 100644
--- a/stack/btu/btu_task.c
+++ b/stack/btu/btu_task.c
@@ -19,6 +19,7 @@
 #define LOG_TAG "bt_btu_task"
 
 #include <assert.h>
+#include <pthread.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>