Darwin: Fix gdb compilation.

Darwin: Fix gdb compilation.

Due to changes introduced by
commit 4d01a485d29732b19743e8b138897f3509e071b0
('struct expression *' -> gdb::unique_xmalloc_ptr<expression>)
compilation is broken on Darwin.

../gdb/darwin-nat-info.c:733:8: error: assigning to 'struct expression *'
from incompatible type
'expression_up' (aka 'std::__1::unique_ptr<expression, gdb::xfree_deleter<expression> >')
expr = parse_expression (exp);

Beside compilation, memory leak was solved as 'make_clean_up' was not called in previous
version.

2016-12-16  Bernhard Heckel  <bernhard.heckel@intel.com>

gdb/Changelog:
	* darwin-nat-info.c (info_mach_region_command): Use expression_up.