[MIPS] Add static_assert that all Fixups are handled in getFixupKind

Summary:
I recently added a new Fixup kind to our fork of LLVM but forgot to add
it to the table in MipsAsmBackend.cpp. With this static_assert the error
would have been caught instead of zero-initializing the array entries for
the new fixups.

Reviewers: sdardis, atanasyan

Reviewed By: atanasyan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D44895

llvm-svn: 328616