Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I7e061f68acdde368a3b63c029b928133646ebff2
diff --git a/services/surfaceflinger/GpuService.cpp b/services/surfaceflinger/GpuService.cpp
index 0c29971..d93f845 100644
--- a/services/surfaceflinger/GpuService.cpp
+++ b/services/surfaceflinger/GpuService.cpp
@@ -27,7 +27,7 @@
 class BpGpuService : public BpInterface<IGpuService>
 {
 public:
-    BpGpuService(const sp<IBinder>& impl) : BpInterface<IGpuService>(impl) {}
+    explicit BpGpuService(const sp<IBinder>& impl) : BpInterface<IGpuService>(impl) {}
 };
 
 IMPLEMENT_META_INTERFACE(GpuService, "android.ui.IGpuService");