make sure to unlock the screenshot's buffer on destruction

this fixes a memory leak on various devices

Bug: 8475134
Change-Id: I25924674c803a0cb22371a95c60664c36acb0340
diff --git a/include/gui/SurfaceComposerClient.h b/include/gui/SurfaceComposerClient.h
index 23655c7..6bf5b47 100644
--- a/include/gui/SurfaceComposerClient.h
+++ b/include/gui/SurfaceComposerClient.h
@@ -170,6 +170,7 @@
 
 public:
     ScreenshotClient();
+    ~ScreenshotClient();
 
     // frees the previous screenshot and capture a new one
     status_t update(const sp<IBinder>& display);