Fix `make -jN trace-cmd gui`

Doing `make -j8 trace-cmd gui` fails like this:

  CMake Error at build/FindTraceCmd.cmake:110 (MESSAGE):

    Could not find libtraceevent!

  Call Stack (most recent call first):
    CMakeLists.txt:24 (include)

Or like this:

  CMake Error at build/FindTraceCmd.cmake:64 (MESSAGE):

    Could not find trace-cmd!

  Call Stack (most recent call first):
    CMakeLists.txt:20 (include)

That's because we need `trace-cmd` to have finished
building before starting to build `gui`.

Link: https://lore.kernel.org/linux-trace-devel/160564780533.18208.2518938894299815863.stgit@toolbox-dario-user-work

Signed-off-by: Dario Faggioli <dfaggioli@suse.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>