IndexEntry: _key should be private

It doesn't have any users outside of the IndexEntry class.

Signed-off-by: Anna Schumaker <Anna@OcarinaProject.net>

include/core/index.h

@@ -16,8 +16,10 @@
  * integer identifiers.  This lets us use a Database as an inverted index.
  */
 class IndexEntry : public DatabaseEntry {
-public:
+private:
 	std::string		_key;    /**< The term stored by this IndexEntry. */
+
+public:
 	std::set<unsigned int>	_values; /**< Integers representing strings that
 					      contain this term. */