rp-pppoe: depend on ppp instead of selecting it

ppp has multiple variants, so selecting one of them introduces a
recursive dependency for any packge selecting it.

Signed-off-by: Felix Fietkau <nbd@nbd.name>

net/rp-pppoe/Makefile

@@ -37,7 +37,7 @@ endef
 define Package/rp-pppoe-common
   $(call Package/rp-pppoe/Default)
   TITLE+= common
-  DEPENDS:=+ppp
+  DEPENDS:=ppp
 endef
 
 define Package/rp-pppoe-common/description