Fix linking problem on Ubuntu 12.04
Fix linking problem on Ubuntu 12.04

Error was:

    /root/.opam/system/lib/mirage-profile/libmProf_unix_stubs.a(time_stubs.o):
In function `stub_mprof_get_monotonic_time':
    /root/.opam/system/build/mirage-profile.0.7.0/_build/unix/time_stubs.c:31:
undefined reference to `clock_gettime'

Hopefully fixes https://github.com/mirage/mirage-skeleton/pull/135

ComposMin/mirage-profile,ComposMin/mirage-profile