Use 'explicit' to avoid incorrect code

Summary:
A single argument constructor could accidentely be used as a type conversion constructor.
Add 'explicit' keyword to avoid this.

Squash with Issue #4: Efficient VARCHAR storage for keys

Test Plan: MTR

Reviewers: jtolmer, hermanlee4

Reviewed By: hermanlee4

Differential Revision: https://reviews.facebook.net/D48069

storage/rocksdb/rdb_datadic.h

@@ -97,7 +97,7 @@ class Stream_reader
   const char* ptr;
   uint len;
 public:
-  Stream_reader(const std::string &str)
+  explicit Stream_reader(const std::string &str)
   {
     len= str.length();
     if (len)
@@ -114,7 +114,7 @@ class Stream_reader
     }
   }
 
-  Stream_reader(const rocksdb::Slice *slice)
+  explicit Stream_reader(const rocksdb::Slice *slice)
   {
     ptr= slice->data();
     len= slice->size();