package/ltrace: do not enable libunwind support for MIPS

Currently libunwind support in ltrace is broken for MIPS. I'm working
with upstream to fix this issue, but it's not yet ready, so let's
disable it by now.

Fixes:

  http://autobuild.buildroot.net/results/79b/79b51941ed57b0564c68112489b03cac39a04e9a/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>