Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I0ce850de85e0de6a5472dd142f738856142db9ec
diff --git a/libs/binder/MemoryDealer.cpp b/libs/binder/MemoryDealer.cpp
index 8739625..a5ba4ac 100644
--- a/libs/binder/MemoryDealer.cpp
+++ b/libs/binder/MemoryDealer.cpp
@@ -126,7 +126,7 @@
         PAGE_ALIGNED = 0x00000001
     };
 public:
-    SimpleBestFitAllocator(size_t size);
+    explicit SimpleBestFitAllocator(size_t size);
     ~SimpleBestFitAllocator();
 
     size_t      allocate(size_t size, uint32_t flags = 0);