gnutls: re-enable gnutls-openssl compat library

This was disabled in the 3.4.x branch by default and is required by
inadyn, fixes:
http://autobuild.buildroot.net/results/51f/51f7e9275c2d23952c7558a126a8843d5476e4b1/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>