spk.mk: Also include files in staging root directory in package.tgz

When testing the package creation with spk/demoservice it appeared
that files in staging/README was not being processed.  This is due
to the "-type d" option which only include directories, thus leaving
files location at root of staging (e.g. target/) to be skipped.

mk/spksrc.spk.mk

@@ -264,7 +264,7 @@ $(DSM_LICENSE_FILE): $(LICENSE_FILE)
 $(WORK_DIR)/package.tgz: icon service
 	$(create_target_dir)
 	@[ -f $@ ] && rm $@ || true
-	(cd $(STAGING_DIR) && find . -mindepth 1 -maxdepth 1 -not -empty -type d | tar cpzf $@ --owner=root --group=root --files-from=/dev/stdin)
+	(cd $(STAGING_DIR) && find . -mindepth 1 -maxdepth 1 -not -empty | tar cpzf $@ --owner=root --group=root --files-from=/dev/stdin)
 
 DSM_SCRIPTS = $(addprefix $(DSM_SCRIPTS_DIR)/,$(DSM_SCRIPT_FILES))