package/python: fix sqlite3 extension without threads

Fix a compiler error when building the sqlite3 extension, when the
toolchain does not have threads.

Fixes:
        http://autobuild.buildroot.net/results/10f/10f268cd55f16ee53f4c5e17e1248b0a1820f6ea/



Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>