Encapsulate the definition of Mutex into glog's internal namespace.

Encapsulate the definition of Mutex into glog's internal namespace.

This will fix the issue a user in Japan reported in his blog.

http://kzk9.net/blog/2009/05/deadlock_with_gflags_and_glog_2.html

According to his description, when he use glog and gflags-1.1, gflags'
code uses glog's Mutex, the Mutex cannot lock the pthread_mutex, and
the program stops.

The bug happens with the combination of gflags-1.1 and glog. It seems
that the issue was caused by incompatibility of Mutex classes. Though I
couldn't reproduce this issue, the reporter of this bug said that
adding namespace fixes the problem and I think it is generally good
idea to use namespace for this kind of common component to avoid
incompatibilities. So, I check this change in now. This patch
will protect glog from future changes on Mutex as well.
--This line, and  those below, will be ignored--

M    src/base/mutex.h

git-svn-id: https://google-glog.googlecode.com/svn/trunk@51 eb4d4688-79bd-11dd-afb4-1d65580434c0