tests/sys/kern/procdesc.c: mark grandchild var in pdopenpid_capmode() as volatile

[kib]

Summary

The variable is written in the child process which shares the address
space with the parent.  The data flow must not be optimized by a compiler.

Fixes:  ddf62c83fc0a ("sys/tests/kern/pdopenpid: pdopenpid(2) is allowed in cap mode")