powerpc/powernv/pci: fix header guard

powerpc/powernv/pci: fix header guard

While looking at -Wundef warnings, the #if CONFIG_EEH stood out as a
possible candidate to convert to #ifdef CONFIG_EEH.

It seems that based on Kconfig dependencies it's not possible to build
this file without CONFIG_EEH enabled, but based on upstream discussion,
it's not clear yet that CONFIG_EEH should be enabled by default.

For now, simply fix the -Wundef warning.

Suggested-by: Nathan Chancellor <nathan@kernel.org>
Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://github.com/ClangBuiltLinux/linux/issues/570
Link: https://lore.kernel.org/lkml/67f6cd269684c9aa8463ff4812c3b4605e6739c3.camel@perches.com/
Link: https://lore.kernel.org/lkml/CAOSf1CGoN5R0LUrU=Y=UWho1Z_9SLgCX8s3SbFJXwJXc5BYz4A@mail.gmail.com/
Link: https://lore.kernel.org/r/20210518204044.2390064-1-ndesaulniers@google.com