musl: add a sys/queue.h implementation

Musl does not provide a 'sys/queue.h' implementation, and this has been
a problem for packages that depend on it.

So lets create a package called netbsd-queue that will install a
'sys/queue.h' in the staging directory when enabled, based on the
NetBSD implementation.

Musl toolchain and external toolchain packages will depend on this
package, so that 'sys/queue.h' will be always installed when compiling
with a musl based toolchain.

Tested on ARM and x86 in the following cases:
  - Buildroot musl toolchain.
  - External musl toolchain without 'sys/queue.h'.
  - External musl toolchain with 'sys/queue.h'.

Fixes:
http://autobuild.buildroot.net/results/24bad2d06ab40024dacf136bee722072d587f84e



And possibly many others.

Signed-off-by: Sergio Prado <sergio.prado@e-labworks.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>