.gitignore: ignore dlntest.dll

This was generated with in-place build on MinGW and not ignored.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60746 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

.gitignore

@@ -154,6 +154,7 @@ lcov*.info
 /ext/*/exts.mk
 
 # /ext/-test-/win32/dln/
+/ext/-test-/win32/dln/dlntest.dll
 /ext/-test-/win32/dln/dlntest.exp
 /ext/-test-/win32/dln/dlntest.lib