libsecret: not available on AVR32, compiler error

libsecret does built on AVR32 due an internal compiler error, and
since it's unlikely that gcc will ever be upgraded on AVR32, we simply
mark this package as not available. The only reverse dependency of
libsecret is webkit, which already cannot be selected on AVR32.

Fixes:

  http://autobuild.buildroot.org/results/2d4/2d41f97378f06d0f3922e078ec0d02549690d4c1/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>