Typo in macOS Sierra macro in configure

sbs reports that there is a typo in the MAC_OS_VERSION_10_12 macro in: https://gitweb.torproject.org/tor.git/tree/configure.ac#n433 (It's missing an 'X_'.)

This has no impact, because we define MAC_OS_VERSION_10_12 if it's not defined (which it never is).

This was introduced in commit 16fcbd21 in tor-0.2.8.10.

Reported by Simone Basso, please credit in changes file.

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