Julia compilation fixes

Summary:
While compilling master, a few failures regarding julia where raised
on my computer:

Scanning dependencies of target cantor_juliaserver
[ 16%] Building CXX object
src/backends/julia/juliaserver/CMakeFiles/cantor_juliaserver.dir/juliaserver.cpp.o
In file included from
/home/tcanabrava/Projects/kdesrc/cantor/src/backends/julia/juliaserver/juliaserver.cpp:22:0:
/usr/include/julia/julia.h:59:27: fatal error: julia_threads.h: No such
file or directory
#include <julia_threads.h>

^
compilation terminated.

Inspecting the FindJulia that's shipped with Cantor, the issue seemed
obvious to me: it doesn't tries to find LibJulia, nor it does add
the julia includes to the include_directories.

Signed-off-by: Tomaz Canabrava <tcanabrava@kde.org>

Differential Revision: https://phabricator.kde.org/D3070