Visibility specifiers should go before return types

[ronawho]

Really minor, but with 1.12 I started seeing warnings for qthread_tos and qthread_bos saying:

src/qthread.c:1885:1: warning: ‘visibility’ attribute ignored on non-class types [-Wattributes]

Looks like visibility specifiers are supposed to go before the return type:

• https://gcc.gnu.org/bugzilla/show_bug.cgi?id=41942
• https://gcc.gnu.org/onlinedocs/gcc/Attribute-Syntax.html#Attribute-Syntax

[ronawho]

As noted in ronawho@1699c31, I only moved the visibility specifiers for qthread_tos() and qthread_bos()

Presumably this change should be made for all of qthreads.