Ignore /mjit_config.h

[Fix GH-1822]

From: hkdnet <satoko.itse@gmail.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62525 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

.gitignore

@@ -198,4 +198,5 @@ lcov*.info
 
 # MJIT
 /rb_mjit_header.h
+/mjit_config.h
 /include/ruby-*/*/rb_mjit_header-*.h