[lldb] Add LLVM include dirs prior to gtest target in standalone build

Move include_directories() declaration before gtest targets are created
in standalone build.  This fixes build failure due to gtest targets
being unable to find LLVM headers, e.g.:

    /var/tmp/portage/dev-util/lldb-16.0.0_pre20221023/work/llvm/utils/unittest/googletest/include/gtest/internal/custom/raw-ostream.h:43:10: fatal error: llvm/ADT/Optional.h: No such file or directory

Differential Revision: https://reviews.llvm.org/D136552