hopefully fix out-of-tree build broken by 26139

diff --git a/applets/Kbuild b/applets/Kbuild
index 8aafe78..6c1c8a3 100644
--- a/applets/Kbuild
+++ b/applets/Kbuild
@@ -13,21 +13,27 @@
 always:= $(hostprogs-y)
 
 # Generated files need additional love
-# NB: __srctree is either empty or "srctree/"
-# using it instead of srctree decreases amount of rebuilds
-# if tree is merely renamed/copied
 
-HOSTCFLAGS_usage.o = -I$(__srctree)include
+# This trick decreases amount of rebuilds
+# if tree is merely renamed/copied
+ifeq ($(src),$(obj))
+srctree_slash =
+else
+srctree_slash = $(srctree)/
+endif
+
+
+HOSTCFLAGS_usage.o = -I$(srctree_slash)include
 
 applets/applets.o: include/usage_compressed.h include/applet_tables.h
 
-applets/usage:         .config $(__srctree)applets/usage_compressed
+applets/usage:         .config $(srctree_slash)applets/usage_compressed
 applets/applet_tables: .config
 
 quiet_cmd_gen_usage_compressed = GEN     include/usage_compressed.h
-      cmd_gen_usage_compressed = $(__srctree)applets/usage_compressed include/usage_compressed.h applets
+      cmd_gen_usage_compressed = $(srctree_slash)applets/usage_compressed include/usage_compressed.h applets
 
-include/usage_compressed.h: applets/usage $(__srctree)applets/usage_compressed
+include/usage_compressed.h: applets/usage $(srctree_slash)applets/usage_compressed
 	$(call cmd,gen_usage_compressed)
 
 quiet_cmd_gen_applet_tables = GEN     include/applet_tables.h