llvm-mc: Fix build breakage caused by r205050.

When LLVM is not built with zlib, nocompression.s will test
for the error message.  But this test case will cause breakage
because the exit code is non-zero.  This commit fix this issue
by adding "not" to the command.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205102 91177308-0d34-0410-b5e6-96231b3b80d8