Don't include undef in cross reference table.

From upstream's 55f7fb8848b6e4bec291724a479e1580d6f407d6.

Change-Id: I54ebc182addbf643bebc78aab03ba1327e24e2e7
Reviewed-on: https://boringssl-review.googlesource.com/2470
Reviewed-by: Adam Langley <agl@google.com>
2 files changed