[LLD][ELF] Follow the common pattern in a message about an undefined vtable symbol.
In most cases, LLD prints its multiline diagnostic messages starting
additional lines with ">>> ". That greatly helps external tools to parse
the output, simplifying combining several lines of the log back into one
message. The patch fixes the only message I found that does not follow
the common pattern.
Differential Revision: https://reviews.llvm.org/D77132