msi.h is generic for all architectures except x86, which has its own
version.  Enabling MSI by adding msi.h to every architecture's Kbuild is
just an additional step which doesn't need to be done.

Make msi.h mandatory in the asm-generic/Kbuild so we don't have to do it
for each architecture.
Suggested-by: NChristoph Hellwig <hch@infradead.org>
Link: https://lore.kernel.org/r/c991669e29a79b1a8e28c3b4b3a125801a693de8.1571983829.git.michal.simek@xilinx.com
Tested-by: Paul Walmsley <paul.walmsley@sifive.com> # build only, rv32/rv64
Signed-off-by: NMichal Simek <michal.simek@xilinx.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Reviewed-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: NWaiman Long <longman@redhat.com>
Acked-by: Paul Walmsley <paul.walmsley@sifive.com> # arch/riscv