Minor: Fix a lint error in cache_test.cc

Summary:
As title. Fix an lint error:

Lint: CppLint Error
Single-argument constructor 'Value(int v)' may inadvertently be used as a type conversion constructor. Prefix the function with the 'explicit' keyword to avoid this, or add an /* implicit */ comment to suppress this warning.

Test Plan: N/A

Reviewers: emayanke, haobo, dhruba

Reviewed By: emayanke

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13401

util/cache_test.cc

@@ -185,7 +185,7 @@ class Value {
  private:
   int v_;
  public:
-  Value(int v) : v_(v) { }
+  explicit Value(int v) : v_(v) { }
 
   ~Value() { std::cout << v_ << " is destructed\n"; }
 };