subprojects: Avoid configure errors due to missing provides

e.g.
WARNING: Subproject 'sdeventplus' did not override 'sdeventplus' dependency and no variable name specified

Change-Id: If1d1ef10ad50517710c2279aae8eb65e24ae15a5
Signed-off-by: Peter Foley <pefoley@google.com>
diff --git a/subprojects/stdplus.wrap b/subprojects/stdplus.wrap
index 00dae65..765ba71 100644
--- a/subprojects/stdplus.wrap
+++ b/subprojects/stdplus.wrap
@@ -1,3 +1,6 @@
 [wrap-git]
 url = https://github.com/openbmc/stdplus
 revision = HEAD
+
+[provide]
+stdplus = stdplus_dep