package/enlightenment: disable ptrace with xtensa and uClibc-ng

uCLibc-ng needs to backport the uClibc commit
de6561f6669308e5d2297589b39a217d4c14df1c to add
PT_GETSIGINFO definition.

This commit is already merged in uClibc-snapshot used by
Buildroot's toolchains for xtensa.

For now, disable ptrace with xtensa uClibc-ng toolchains.

Thanks to Gergely Imreh for the initial investigation and
patch:
http://patchwork.ozlabs.org/patch/470073/

Fixes:
http://autobuild.buildroot.net/results/77b/77b82e5b437d0778b3f6a3813f2c0a5c39b52db9



Reported-by: Gergely Imreh <imrehg@gmail.com>
Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>