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

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.

Reviewed by:	markj
Fixes:	ddf62c83fc ("sys/tests/kern/pdopenpid: pdopenpid(2) is allowed in cap mode")
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D59282

tests/sys/kern/procdesc.c

@@ -372,7 +372,8 @@ ATF_TC_BODY(pdopenpid_child, tc)
 ATF_TC_WITHOUT_HEAD(pdopenpid_capmode);
 ATF_TC_BODY(pdopenpid_capmode, tc)
 {
-	pid_t child, grandchild;
+	pid_t child;
+	volatile pid_t grandchild;
 
 	ATF_REQUIRE_FEATURE("security_capability_mode");