[llvm-objcopy][COFF] Fix test for debug dir presence

If the number of directories was 6 (equal to the DEBUG_DIRECTORY
index), patchDebugDirectory() was run even though the debug directory
is actually the 7th entry. Use <= in the comparison to fix that.

This fixes https://llvm.org/PR51243

Differential Revision: https://reviews.llvm.org/D106940

Reviewed by: jhenderson