lib/libzstd: add an io compartmenet

This isn't a conventional IO interface, but rather some debugging code
that is always compiled it.  Compartmentalize it anyway to limit the
blast radius of bugs elsewhere in the library.

Author: brooksdavis

lib/libzstd/Compartments.json

@@ -0,0 +1,11 @@
+{
+    "compartments": {
+	"io": {
+	    "files": [
+		"cover.*o",
+		"fastcover.*o",
+		"zdict.*o"
+            ]
+        }
+    }
+}

lib/libzstd/Makefile

@@ -42,6 +42,8 @@ LIBADD=	pthread
 # explicitly disable assembly.
 CFLAGS+=	-DZSTD_DISABLE_ASM
 
+COMPARTMENT_POLICY=	${.CURDIR}/Compartments.json
+
 PRIVATELIB=	yes
 PACKAGE=	runtime