signal.c: include ucontext.h

* signal.c: include ucontext.h for ucontext_t.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48830 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-12-14

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48831 b2dd03c8-39d4-4d8f-98ff-823fe69b080e