Use proper create_makefile

Using create_makefile('liboga/liboga') will compile liboga.so into
path-to-gem/lib/liboga/ and therefore require_relative in oga.rb will fail.
Therefore the right parameter for create_makefile is 'liboga' ->
path-to-gem/lib/liboga.so

ext/c/extconf.rb

@@ -10,4 +10,4 @@ else
   $CFLAGS << ' -O3'
 end
 
-create_makefile('liboga/liboga')
+create_makefile('liboga')