Add SuppressWarnings for FutureReturnValueIgnored

https://errorprone.info/bugpattern/FutureReturnValueIgnored

The submitted task logs the error in case it fails.

Change-Id: I280442ddb8046da47aeb79d005886f65d2e3067b
Signed-off-by: Masaya Suzuki <masayasuzuki@google.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/GC.java

@@ -244,6 +244,7 @@ public class GC {
 	 *             parsed
 	 */
 	// TODO(ms): in 5.0 change signature and return Future<Collection<PackFile>>
+	@SuppressWarnings("FutureReturnValueIgnored")
 	public Collection<PackFile> gc() throws IOException, ParseException {
 		if (!background) {
 			return doGc();