[musl] pthread_getname_np implementation

[musl@schoepfer.info]

Hi all,

i'm new to this list.

I'm trying to compile qt5-5.15.2 with gcc10.2 and musl-1.2.1, and after 
some hours compiling i get:

platform/default/thread.cpp: In function ‘std::string 
mbgl::platform::getCurrentThreadName()’:
platform/default/thread.cpp:14:5: error: ‘pthread_getname_np’ was not 
declared in this scope; did you mean ‘pthread_setname_np’?
    14 |     pthread_getname_np(pthread_self(), name, sizeof(name));
       |     ^~~~~~~~~~~~~~~~~~
       |     pthread_setname_np

I found this thread about musl/pthread_getname_np 
https://www.openwall.com/lists/musl/2019/07/09/2 ,
which says
"Based on the previous discussion of this and others' comments, I think
adding it[pthread_getname_np] is probably the right thing to do. I'll 
look at it after
rolling the release. Ping the list again if I don't get around to it
soon."
So here is the ping :-)

If there's another way around to get qt5 to compile, that's of course 
also ok for me.

Thanks,

Johannes