package/lttng-libust: fix build with musl

package/lttng-libust: fix build with musl

Backport two patches from upstream to fix build with musl.

Fixes:
    http://autobuild.buildroot.org/results/dcb/dcb6c3aadf6f955016cf3c3f9b917cbec6d29342/
    http://autobuild.buildroot.org/results/deb/deb80eea5ceae3e7f8e0549596dce2fb6a1e7596/
    http://autobuild.buildroot.org/results/91e/91e64477864825c39911e3f3d0a2459f2987c209/
    [...and a lot more...]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Philippe Proulx <eeppeliteloop@gmail.com>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>