targets/graw-xlib: Add missing dep_x11

Required to build Mesa on macOS with

-Dbuild-tests=true -Dglx=gallium-xlib

Without this change, the build fails with

In file included from ../src/gallium/targets/graw-xlib/graw_xlib.c:8: ../src/gallium/include/frontend/xlibsw_api.h:5:10: fatal error: 'X11/Xlib.h' file not found #include <X11/Xlib.h>

With brew sh X11 is found but linking fails due to llvm-ar missing in the path. That issue appears to be unrelated to this missing dependency. X11 is installed via XQuartz, so Homebrew should not be required.

Cc @jeremyhu