btrfs-progs: fix static build with gettext

gettext needs -pthread. Incidentally this change was also made in the package
Makefile itself for a different reason.

Also, drop -lm to match upstream change.

Fixes:
http://autobuild.buildroot.net/results/a53/a53f31355e955c734010922b47084605e13708ef/
http://autobuild.buildroot.net/results/43d/43deeeda31d9750b37e1f1058c3313a387843bee/



and more.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>