extmk.rb: fix extstatic

* ext/extmk.rb: fix variable name for --extstatic option.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45886 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/extmk.rb

@@ -523,7 +523,7 @@ Dir::chdir('ext')
 hdrdir = $hdrdir
 $hdrdir = ($top_srcdir = relative_from(srcdir, $topdir = "..")) + "/include"
 exts.each do |d|
-  $static = $force_static ? true : $static_ext[target]
+  $static = $force_static ? true : $static_ext[d]
 
   if $ignore or !$nodynamic or $static
     extmake(d) or abort