Fix ambiguous mutex reference by explicitly using std::mutex

[psumbera]

On illumos/Solaris you get two different mutex symbols:

C++ standard library

std::mutex

System header

/usr/include/sys/mutex.h
typedef struct mutex { ... } mutex;

Because libmemcached includes system networking headers before , the unqualified name mutex becomes ambiguous once is included.

GCC 15 is stricter and now errors out instead of picking one.