selftests: splice: fix clean target to not remove default_file_splice_read.sh

splice clean target removes the shell script default_file_splice_read.sh
that runs the splice test. Fix it to not remove this file.
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/splice/Makefile

@@ -5,4 +5,4 @@ all: $(TEST_PROGS) $(EXTRA)
 include ../lib.mk
 
 clean:
-	rm -fr $(TEST_PROGS) $(EXTRA)
+	rm -fr $(EXTRA)