libprocessgroup: add -Wexit-time-destructors.

Exit time destructors already caused trouble (see
dba6d44376a3301acfb8fd7e7c5159b25736b09e), and without this warning,
it's too easy to reintroduce issues.

Bug: 129023204
Test: builds
Change-Id: If5fb4d0c11c74ba4971bb218059c6d37edeaf4df
1 file changed