Fix reference to libX11.so

Closes #19811 (closed)

Changed the refs to both libX11.so->libX11.so.6 in the search and the warning, and updated the search for libXcursor.so->libXcursor.so.1 (the warning already referenced the .1).

Not sure this is the right approach but seems reasonable given what the non-dev packages actually ship. Feel free to close if there's a better solution!