internal.h: supperss warning

* internal.h (SIGNED_INTEGER_MAX): supperss warning C4146 on VC6.
  seems a logical ORed expression becomes unsigned.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41762 b2dd03c8-39d4-4d8f-98ff-823fe69b080e