Make the mkmf generated Makefile produce a chipmunk.so in the lib directory

[willaerk]

Came across this bug when building and installing a gem from the latest commit in master. The .so with
the actual chipmunk library is put in lib/chipmunk/chipmunk.so instead of lib/chipmunk.so.
This results in an error trying to import it:

> require 'chipmunk'
Traceback (most recent call last):
        7: from /usr/bin/irb:23:in `<main>'
        6: from /usr/bin/irb:23:in `load'
        5: from /usr/lib/ruby/gems/2.7.0/gems/irb-1.2.1/exe/irb:11:in `<top (required)>'
        4: from (irb):2
        3: from (irb):2:in `require'
        2: from /home/willaerk/Documents/Games/foobar/vendor/bundle/ruby/2.7.0/gems/chipmunk-6.1.3.4/lib/chipmunk.rb:1:in `<top (required)>'
        1: from /home/willaerk/Documents/Games/foobar/vendor/bundle/ruby/2.7.0/gems/chipmunk-6.1.3.4/lib/chipmunk.rb:1:in `require_relative'
LoadError (cannot load such file -- /home/willaerk/Documents/Games/foobar/vendor/bundle/ruby/2.7.0/gems/chipmunk-6.1.3.4/lib/chipmunk.so)

It is probably introduced by this change:

f161f16#diff-f91cdc8291e5010cab7f399b05dcfe6c13ab62cd6ca017d1938a8ff38eaad419L1-R1

This change makes sure the .so is put in lib/chipmunk.so when installing the gem.