WIP: lttng-modules: new package

FIXME: build errors out (and clean-up expression when it works).
On Ubuntu, this works just fine:

  cd lttng-modules-2.2.0 && make
  [...]
    LD [M]  /home/bfo/lttng-modules-2.2.0/lib/lttng-lib-ring-buffer.o
  /home/bfo/lttng-modules-2.2.0/probes/Makefile:70: Files /usr/src/linux-headers-3.5.0-34-generic/fs/ext3/*.h not found. Probe
  "ext3" is disabled. Use full kernel source tree to enable it.
  /home/bfo/lttng-modules-2.2.0/probes/Makefile:131: Files /usr/src/linux-headers-3.5.0-34-generic/fs/btrfs/*.h not found. Probe
  "btrfs" is disabled. Use full kernel source tree to enable it.
  /home/bfo/lttng-modules-2.2.0/probes/Makefile:147: Files /usr/src/linux-headers-3.5.0-34-generic/fs/ext4/*.h not found. Probe
  "ext4" is disabled. Use full kernel source tree to enable it.
  [...SUCCESS...]

But on NixOS it doesn't say "ext3/*h not found" but later errors out:

  building /tmp/nix-build-lttng-modules-2.2.0.drv-0/lttng-modules-2.2.0/probes/lttng-probe-ext3.o
    CC [M]  /tmp/nix-build-lttng-modules-2.2.0.drv-0/lttng-modules-2.2.0/probes/lttng-probe-ext3.o
  In file included from /tmp/nix-build-lttng-modules-2.2.0.drv-0/lttng-modules-2.2.0/probes/../instrumentation/events/lttng-module/../../../probes/lttng-events.h:759:0,
                   from /tmp/nix-build-lttng-modules-2.2.0.drv-0/lttng-modules-2.2.0/probes/../instrumentation/events/lttng-module/../../../probes/define_trace.h:148,
                   from /tmp/nix-build-lttng-modules-2.2.0.drv-0/lttng-modules-2.2.0/probes/../instrumentation/events/lttng-module/ext3.h:868,
                   from /tmp/nix-build-lttng-modules-2.2.0.drv-0/lttng-modules-2.2.0/probes/lttng-probe-ext3.c:48:
  /tmp/nix-build-lttng-modules-2.2.0.drv-0/lttng-modules-2.2.0/probes/../instrumentation/events/lttng-module/../../../probes/../instrumentation/events/lttng-module/ext3.h:
  In function '__event_probe__ext3__page_op':
  /tmp/nix-build-lttng-modules-2.2.0.drv-0/lttng-modules-2.2.0/probes/../instrumentation/events/lttng-module/../../../probes/../instrumentation/events/lttng-module/ext3.h:240:1:
  error: dereferencing pointer to incomplete type
  /tmp/nix-build-lttng-modules-2.2.0.drv-0/lttng-modules-2.2.0/probes/../instrumentation/events/lttng-module/../../../probes/../instrumentation/events/lttng-module/ext3.h:240:1:
  error: dereferencing pointer to incomplete type
  /tmp/nix-build-lttng-modules-2.2.0.drv-0/lttng-modules-2.2.0/probes/../instrumentation/events/lttng-module/../../../probes/../instrumentation/events/lttng-module/ext3.h:240:1:
  error: dereferencing pointer to incomplete type
  /tmp/nix-build-lttng-modules-2.2.0.drv-0/lttng-modules-2.2.0/probes/../instrumentation/events/lttng-module/../../../probes/../instrumentation/events/lttng-module/ext3.h:
  In function '__event_probe__ext3_invalidatepage':
  /tmp/nix-build-lttng-modules-2.2.0.drv-0/lttng-modules-2.2.0/probes/../instrumentation/events/lttng-module/../../../probes/../instrumentation/events/lttng-module/ext3.h:298:1:
  error: dereferencing pointer to incomplete type
  /tmp/nix-build-lttng-modules-2.2.0.drv-0/lttng-modules-2.2.0/probes/../instrumentation/events/lttng-module/../../../probes/../instrumentation/events/lttng-module/ext3.h:298:1:
  error: dereferencing pointer to incomplete type
  /tmp/nix-build-lttng-modules-2.2.0.drv-0/lttng-modules-2.2.0/probes/../instrumentation/events/lttng-module/../../../probes/../instrumentation/events/lttng-module/ext3.h:298:1:
  error: dereferencing pointer to incomplete type
  make[3]: *** [/tmp/nix-build-lttng-modules-2.2.0.drv-0/lttng-modules-2.2.0/probes/lttng-probe-ext3.o] Error 1
  make[2]: *** [/tmp/nix-build-lttng-modules-2.2.0.drv-0/lttng-modules-2.2.0/probes] Error 2
  make[1]: *** [_module_/tmp/nix-build-lttng-modules-2.2.0.drv-0/lttng-modules-2.2.0] Error 2
  make[1]: Leaving directory `/nix/store/bqsgm4hdacwmkqgpw51w8fkva9klas44-linux-3.2.47/lib/modules/3.2.47/build'
  make: *** [default] Error 2
  builder for `/nix/store/s23i64191z68vcbzigss8w0qrhhhcggl-lttng-modules-2.2.0.drv' failed with exit code 2
  error: build of `/nix/store/s23i64191z68vcbzigss8w0qrhhhcggl-lttng-modules-2.2.0.drv' failed

Add linux kernel modules needed to do kernel level tracing with LTTng.