Appease autoconf build since X86Disassembler.c has been disappeared in r206717.

It can be reverted a few days later, after X86Disassembler.d is updated not to contain "X86Disassembler.c".

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206758 91177308-0d34-0410-b5e6-96231b3b80d8