powerpc: convert hvconsole.c to export.h ; fix implicit use of errno.h

This file is only exporting symbols and so should use export.h
and not module.h header.  But in doing the conversion, we will
uncover that it was implicitly using errno.h via module.h:

  CC      arch/powerpc/platforms/pseries/hvconsole.o
arch/powerpc/platforms/pseries/hvconsole.c: In function 'hvc_put_chars':
arch/powerpc/platforms/pseries/hvconsole.c:77: error: 'EIO' undeclared (first use in this function)

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>