powerpc/xive: Define xive_do_source_eoi as static

Sparse shows that xive_do_source_eoi() file is defined without any
declaration, thus, it should be a static function.

	arch/powerpc/sysdev/xive/common.c:312:6: warning: symbol 'xive_do_source_eoi' was not declared. Should it be static?

This patch simply turns this symbol into static.

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>