FindRuby: Restore compatibility variable RUBY_VENDORLIB_DIR

Since commit e672db628b (FindRuby: Rename variables to match case of
module name, 2020-03-11, v3.18.0-rc1~546^2), the result variables named
with the old `RUBY_` prefix are provided by compatibility code that maps
from the new `Ruby_` prefix variables.  This variable was accidentally
left out.

Fixes: #21369

Modules/FindRuby.cmake

@@ -515,6 +515,7 @@ foreach(Camel
     Ruby_SITELIB_DIR
     Ruby_HAS_VENDOR_RUBY
     Ruby_VENDORARCH_DIR
+    Ruby_VENDORLIB_DIR
 
     )
     string(TOUPPER ${Camel} UPPER)