Fix build rules for LLVM_WITH_Z3 after D95727

Started to see build errors like this

../lib/Support/Z3Solver.cpp:19:10: fatal error: 'z3.h' file not found
#include <z3.h>
         ^~~~~~
1 error generated.

after commit 43ceb74e.

The -isystem path to the Z3_INCLUDE_DIR wen't missing in the compile
commands. No idea why target_include_directories stopped working with
that commit, but using include_directories seem to work better.