gthread: use inline keyword for _get_impl() functions
gthread: use inline keyword for _get_impl() functions

Give compiler a hint that these should be inlined,
which doesn't seem to happen by default with -O2.
Yields 5% speedup in artificial benchmarks, and
1% speedup in a real-world test case doing a lot
of mutex locking and unlocking.

https://bugzilla.gnome.org/show_bug.cgi?id=730807

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