package/mpd: add patch to fix musl gcc5 build issue

Add a patch from the Alpine Linux project [1] to fix a musl build issue with
gcc 5:

```
error: temporary of non-literal type ‘pthread_mutex_t’ in a constant expression
  constexpr PosixMutex():mutex(PTHREAD_MUTEX_INITIALIZER) {}
```

Problem has been reported by the Alpine team upstream and was closed by the MPD
maintainer with WONTFIX:
http://bugs.musicpd.org/view.php?id=4387
http://bugs.musicpd.org/view.php?id=4110

However...

POSIX does not permit using PTHREAD_COND_INITIALIZER except for static
initialization, and certainly does not permit using it as a value.

Also POSIX does not specify the type of the object (it's opaque) so if
there are any types for which their code would be invalid C++, then their
code is invalid.

Also, volatile in the type is necessary. without that, LTO can break the code.

[1]
http://git.alpinelinux.org/cgit/aports/log/main/mpd/musl-gcc5-fixes.patch



Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>