External linkage is the default for function declarations in C++.

This also fixes ClangTidy errors generated by removing the "extern"
keyword as described above.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=188730416

git-svn-id: https://leveldb.googlecode.com/svn/trunk@23 62dab493-f737-651d-591e-8d6aee1b9529

git-svn-id: https://leveldb.googlecode.com/svn/trunk@21 62dab493-f737-651d-591e-8d6aee1b9529

git-svn-id: https://leveldb.googlecode.com/svn/trunk@20 62dab493-f737-651d-591e-8d6aee1b9529

git-svn-id: https://leveldb.googlecode.com/svn/trunk@2 62dab493-f737-651d-591e-8d6aee1b9529