fitblk: invert kernel version dependency

Allow building fitblk with Linux 6.6 by depending on !LINUX_5_15
instead of LINUX_6_1.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

package/utils/fitblk/Makefile

@@ -16,7 +16,7 @@ define Package/fitblk
   SECTION:=base
   CATEGORY:=Base system
   TITLE:=fitblk firmware release tool
-  DEPENDS:=@LINUX_6_1
+  DEPENDS:=@!LINUX_5_15
 endef
 
 define Package/fitblk/description