subprojects: Update to HEAD

Change-Id: Idbc173920abc588d35be56fa7cd3171cac6673cf
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/subprojects/span-lite.wrap b/subprojects/span-lite.wrap
index f7bcddb..ee0f0db 100644
--- a/subprojects/span-lite.wrap
+++ b/subprojects/span-lite.wrap
@@ -1,3 +1,3 @@
 [wrap-git]
 url = https://github.com/martinmoene/span-lite
-revision = v0.7.0
+revision = HEAD