Readd pthread_mutex.h include on ngx_fetch.cc.
Readd pthread_mutex.h include on ngx_fetch.cc.

Messed up the merge of 0290f52a8892c87eed4103e1990a07d0838fd05e originally, add the include back.

wanrui/ngx_pagespeed,0x0mar/ngx_pagespeed,wanrui/ngx_pagespeed,digideskio/ngx_pagespeed,digideskio/ngx_pagespeed,0x0mar/ngx_pagespeed