Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I0ce850de85e0de6a5472dd142f738856142db9ec
diff --git a/libs/binder/BufferedTextOutput.cpp b/libs/binder/BufferedTextOutput.cpp
index 1339a67..a2443c0 100644
--- a/libs/binder/BufferedTextOutput.cpp
+++ b/libs/binder/BufferedTextOutput.cpp
@@ -34,7 +34,7 @@
 
 struct BufferedTextOutput::BufferState : public RefBase
 {
-    BufferState(int32_t _seq)
+    explicit BufferState(int32_t _seq)
         : seq(_seq)
         , buffer(NULL)
         , bufferPos(0)