llvm-isel-fuzzer: Stop including FuzzerInterface.h

All this does is forward declare the interface functions (and make
sure that they're `extern "C"`), but since we're using libFuzzer from
the toolchain it doesn't make sense to include the local copy of the
interface.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312195 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed