ltrace: enable for mips/mipsel

ltrace was disabled for MIPS due to a build failure on this
architecture. Now that problem has been fixed so we can enable this
package again.

Related:
http://git.buildroot.net/buildroot/commit/?id=dfaa18af00dfcff71a0159adaafb0880e58fa6a1
http://anonscm.debian.org/cgit/collab-maint/ltrace.git/commit/?id=a8ff0ff2b6e6c229f8a9242828fb47f1a5a78967



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