selftests/signedoff-check.sh: fix use of return with exit

When this test used to be a function, it was OK to use "return",
but now that it's a script (and the code is not wrapped in a
function) that is invalid shell syntax.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

selftests/signedoff-check.sh

@@ -4,5 +4,5 @@ AUTHOR="$(git log --no-merges -1 --pretty='format:%aN <%aE>')"
 git log --no-merges -1 --pretty=format:%B | grep "Signed-off-by: $AUTHOR"
 if [ $? != 0 ]; then
     echo "The commit message does not contain author's signature (Signed-off-by: $AUTHOR)"
-    return 1
+    exit 1
 fi