gnutls: revert bump

Revert bump to 3.2.7 from commit 8238e744
It causes build issues because of an internal gnulib bump:
http://autobuild.buildroot.net/results/48d/48d5c6364fec55e84309b2fb6986f206699f1800/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>