Fix luajit includedir when cross-compiling

The use of --variable=includedir for luajit doesn't include the root-directory when cross-compiling.

Example with Gentoo (from meson-logs/meson-log.txt):

Called: `/usr/bin/aarch64-unknown-linux-gnu-pkg-config --variable=includedir luajit` -> 0
stdout:
/usr/include/luajit-2.1
-----------
Got pkg-config variable includedir : /usr/include/luajit-2.1

modules/meson.build:37:14: ERROR: Include dir /usr/include/luajit-2.1 does not exist.

After the patch applied (from meson-info/intro-dependencies.json):

  {
    "name": "luajit",
    "type": "pkgconfig",
    "version": "2.1.1770848219",
    "compile_args": [
      "-I/usr/aarch64-unknown-linux-gnu/usr/include/luajit-2.1"
    ],
    "link_args": [
      "/usr/aarch64-unknown-linux-gnu/usr/lib64/libluajit-5.1.so"
    ],
    "include_directories": [],
    "sources": [],
    "extra_files": [],
    "dependencies": [],
    "depends": [],
    "meson_variables": [
      "luajit"
    ]
  }

Signed-off-by: Nicolas PARLANT nicolas.parlant@parhuet.fr