src: Allow for stdplus to be vendored

Change-Id: I0ade83798e1458e31b011119f95fc17c30617153
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/subprojects/span-lite.wrap b/subprojects/span-lite.wrap
new file mode 100644
index 0000000..f7bcddb
--- /dev/null
+++ b/subprojects/span-lite.wrap
@@ -0,0 +1,3 @@
+[wrap-git]
+url = https://github.com/martinmoene/span-lite
+revision = v0.7.0