[LLD][ELF] - Update test case to check the error message reported. NFC.

This updates znotext-plt-relocations-protected.s test case
(adds a file symbol) to check how the error message is built.

It allows to cover/test the following line:
https://github.com/llvm-mirror/lld/blob/master/ELF/Relocations.cpp#L79

llvm-svn: 339550