licenses: add LGPL license for GEOS

[otan]

Release note: None

[cockroach-teamcity]

This change is 

[bdarnell]

The filename should be LGPL.txt, not LGPL-libgeos.txt (the reason we have per-project files like BSD-*.txt is that these licenses are not actually identical, but the LGPL is more standardized than the BSD/MIT families)

I wonder if we should include a more explicit mapping of component to license in this directory (especially when it comes to the line between copyleft and the permissive licenses that cover nearly all of our dependencies).

[otan]

thanks! changes made!

bors r=jlinder,bdarnell

[otan]

I wonder if we should include a more explicit mapping of component to license in this directory (especially when it comes to the line between copyleft and the permissive licenses that cover nearly all of our dependencies).

probably more worthwhile at some point!

[craig]

Build succeeded:

• GitHub CI (Cockroach)