update krb5 external on windows to 1.12.1 when building with VS 2012
the 1.12.1 externald builds properly with VS 2012, wereas the external
we were using before (krb5-1.10) was built wtih VS 2010 and does not
build with VS 2012.
This commit fixes the problem we had been having where the MSI from the Windows7
build machine would not install.
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