gthread-posix: Don't use gslice allocated GRecMutex
gthread-posix: Don't use gslice allocated GRecMutex

This leads to problems during cleanup, and seems strange
to have locks defined in terms of things that need locking.

https://bugzilla.gnome.org/show_bug.cgi?id=711753

tamaskenez/glib,Distrotech/glib,mzabaluev/glib,krichter722/glib,tamaskenez/glib,johne53/MB3Glib,krichter722/glib,tchakabam/glib,gale320/glib,endlessm/glib,johne53/MB3Glib,ieei/glib,gale320/glib,lukasz-skalski/glib,cention-sany/glib,MathieuDuponchelle/glib,lukasz-skalski/glib,johne53/MB3Glib,Distrotech/glib,ieei/glib,tamaskenez/glib,tchakabam/glib,lukasz-skalski/glib,MathieuDuponchelle/glib,endlessm/glib,gale320/glib,MathieuDuponchelle/glib,cention-sany/glib,ieei/glib,tamaskenez/glib,lukasz-skalski/glib,mzabaluev/glib,endlessm/glib,johne53/MB3Glib,MathieuDuponchelle/glib,Distrotech/glib,ieei/glib,krichter722/glib,tchakabam/glib,gale320/glib,cention-sany/glib,gale320/glib,MathieuDuponchelle/glib,mzabaluev/glib,cention-sany/glib,krichter722/glib,johne53/MB3Glib,Distrotech/glib,mzabaluev/glib,endlessm/glib,tchakabam/glib,Distrotech/glib,tamaskenez/glib,lukasz-skalski/glib,tchakabam/glib,endlessm/glib,krichter722/glib,cention-sany/glib,johne53/MB3Glib,ieei/glib,mzabaluev/glib