base: sync: include struct seq_file to fix compile warning

Fixes missing declaration for struct seq_file.

Change-Id: Id77c47fad3bbee56d0dbe4a85f56f6f388e686d6
Signed-off-by: Jeff Boody <jboody@codeaurora.org>
diff --git a/include/linux/sync.h b/include/linux/sync.h
index 943f414..4c00f04 100644
--- a/include/linux/sync.h
+++ b/include/linux/sync.h
@@ -24,6 +24,7 @@
 struct sync_timeline;
 struct sync_pt;
 struct sync_fence;
+struct seq_file;
 
 /**
  * struct sync_timeline_ops - sync object implementation ops