ltrace: disable on mips/mipsel

ltrace currently doesn't build on mips/mipsel, and it's an upstream
issue that has been reported. Until it get fixed, let's disable ltrace
for mips/mipsel.

Fixes:

  http://autobuild.buildroot.org/results/43a/43a8fc7075f52eab74ebfee4c9f25dd2b886e75e/



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