nettle: fix missing link to libm

nettle-benchmark uses ldexp without explicitly linking to libm, fixes:
http://autobuild.buildroot.net/results/c8e64fabcb98b1ff4bf737e2181b7cf2ae731550



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>