[spotbugs] Fix incorrect lazy initialization in SystemReader

This fixes two warnings of type LI_LAZY_INIT_STATIC.

Change-Id: I26a7a48aed9d0a0547e908a56b7014a7620fadd8
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/util/SystemReader.java

@@ -56,9 +56,9 @@ public abstract class SystemReader {
 
 	private static final SystemReader DEFAULT;
 
-	private static Boolean isMacOS;
+	private static volatile Boolean isMacOS;
 
-	private static Boolean isWindows;
+	private static volatile Boolean isWindows;
 
 	static {
 		SystemReader r = new Default();