fix accidental implicit matches in elf-disassemble-symbol-labels-rel.test

llvm/test/tools/llvm-objdump/X86/elf-disassemble-symbol-labels-rel.test
uses --implicit-check-not to verify that certain patterns do not occur
in llvm-objdump's output, except in places where they are explicitly
checked. Unfortunately, the patterns are generic enough that they may
be part of the file name which is also output by llvm-objdump. This
change matches the line with the filename explicitly so that the
implicit patterns are not applied to it.

llvm-svn: 361563