Fix r359229 which tried to fix r359159...

When r359229 added noexcept to the declaration of `~mutex`, it didn't
add it to the definition which caused -Wimplicit-exception-spec-mismatch
to fire. This just adapts the definition to agree with the declaration.

git-svn-id: 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/src/mutex.cpp b/src/mutex.cpp
index 6bb44e0..cecb89b 100644
--- a/src/mutex.cpp
+++ b/src/mutex.cpp
@@ -19,7 +19,7 @@
 const try_to_lock_t try_to_lock = {};
 const adopt_lock_t  adopt_lock = {};
+mutex::~mutex() _NOEXCEPT