[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

llvm/lib/MC/XCOFFObjectWriter.cpp

@@ -416,7 +416,7 @@ void XCOFFObjectWriter::writeSymbolName(const StringRef &SymbolName) {
     W.write<int32_t>(0);
     W.write<uint32_t>(Strings.getOffset(SymbolName));
   } else {
-    char Name[XCOFF::NameSize];
+    char Name[XCOFF::NameSize+1];
     std::strncpy(Name, SymbolName.data(), XCOFF::NameSize);
     ArrayRef<char> NameRef(Name, XCOFF::NameSize);
     W.write(NameRef);