lockdev: fix build with the musl C library

When the musl C library is used, the __GNU_LIBRARY__ symbol is not
defined, so the lockdev code doesn't know what to do. To work around
this, we explicitly pass __GNU_LIBRARY__ when building with musl, this
musl provides what's needed for lockdev to build, as if we were
building against glibc.

Fixes:

  http://autobuild.buildroot.org/results/9db/9db4ab40955d3af0027e141245d73ee6c614fb1f/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>