minios: do not expose #define current to applications

minios: do not expose #define current to applications

Currently the minios headers do this:
 #define current get_current()
Obviously when porting general code to this environment, this can
cause problems !

The attached patch arranges for this only to be done if
 #define __MINIOS__
is declared, which is set up by the makefile in extras/mini-os.

Suppressing the namespace pollution is necessary to get recent
upstream qemu's to compile, since they (quite properly) use `current'
as an ordinary identifier.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>