ui: tell ESLint to be strict in check target

the .lint-incremental target, which is implicitly used by the install
target, is still more forgiving to allow faster "change, build, test"
iteration when developing.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

www/Makefile

@@ -83,7 +83,7 @@ js/proxmox-backup-gui.js: .lint-incremental js OnlineHelpInfo.js ${JSSRC}
 
 .PHONY: check
 check:
-	eslint ${JSSRC}
+	eslint --strict ${JSSRC}
 	touch ".lint-incremental"
 
 .lint-incremental: ${JSSRC}