Add PROVIDES to all provider classes

When multiple recipes provide a package PREFERRED_PROVIDER
is used to pick one.  However bitbake doesn't look at
RPROVIDES when it figures this out.
diff --git a/meta-phosphor/classes/obmc-phosphor-system-mgmt.bbclass b/meta-phosphor/classes/obmc-phosphor-system-mgmt.bbclass
index 4939d2f..07c89c4 100644
--- a/meta-phosphor/classes/obmc-phosphor-system-mgmt.bbclass
+++ b/meta-phosphor/classes/obmc-phosphor-system-mgmt.bbclass
@@ -1,3 +1,4 @@
 # Common code for recipes that implement Phosphor OpenBMC system management.
 
 RPROVIDES_${PN} += "virtual/obmc-phosphor-system-mgmt"
+PROVIDES += "virtual/obmc-phosphor-system-mgmt"