package/libsigrok: fix musl build

Fixes
http://autobuild.buildroot.net/results/afc/afcea2068d5b0ba707aa90339401550602103ca7/
http://autobuild.buildroot.net/results/a4d/a4dab61fb39dd642f85fad20972a0a4572af9a73/


and others

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>