[llvm-objcopy] Append '\n' to warning messages

Currently the warning message of `llvm-strip %t.o %t.o` does not include
the trailing newline. Fix this by appending a '\n'.

This is the only warning llvm-objcopy and llvm-strip can issue.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D66475

llvm-svn: 369391