jansson: needs math library

libjansson.so doesn't list libm in its NEEDED header. Fixes the following
nftables config time failure:

.../usr/lib/libjansson.so.4: undefined reference to `__isnan'
.../usr/lib/libjansson.so.4: undefined reference to `__isinf'

Fixes:
http://autobuild.buildroot.net/results/152/1525d7a3e1d1fcf35858962251c0b69a5e1b64db/



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