x86: add missing PAGE_ALIGN macro from page.h

The PAGE_ALIGN macro is present in all other page.h headers, including
the generic one.

This patch adds the missing PAGE_ALIGN macro to ib/x86/asm/page.h

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <20200622162141.279716-3-imbrenda@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>