test: fix FTBS on missing xlib includes on NetBSD

When X11 isn't installed directly at /usr hierarchy (eg. NetBSD uses /usr/X11R7/), build breaks:

../test/list.c:31:10: fatal error: X11/Xlib.h: No such file or directory 31 | #include <X11/Xlib.h> | ^~~~~~~~~~~~

Needs explicitly dependency on libX11, so the include path is added.

Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net