Use etc instead of .so for broader compatibility

The use of `etc.so` here requires that etc is always implemented
as a C extension on-disk. However at least one impl – JRuby –
currently implements it as an internal extension, loaded via a
Ruby script. This require should simply use the base name of the
library, `etc`, to allow Ruby-based implementations to load as
well.

lib/pp.rb

@@ -424,7 +424,7 @@ def pretty_print(q) # :nodoc:
 class File < IO # :nodoc:
   class Stat # :nodoc:
     def pretty_print(q) # :nodoc:
-      require 'etc.so'
+      require 'etc'
       q.object_group(self) {
         q.breakable
         q.text sprintf("dev=0x%x", self.dev); q.comma_breakable