[NFC][AIX][XCOFF] fixed compile warning on the strncpy.

SUMMARY:
There is warning when compile the file XCOFFObjectWriter.cpp
/srv/llvm-buildbot-srcatch/llvm-build-dir/openmp-gcc-x86_64-linux-debian/llvm.src/llvm/lib/MC/XCOFFObjectWriter.cpp:414:17: warning: 'char* strncpy(char*, const char*, size_t)' specified bound 8 equals destination size [-Wstringop-truncation]

The patch fixed the warning.

Reviewer: daltenty

Differential Revision: https://reviews.llvm.org/D71119