fs/littlefs/Make.defs: fix a littlefs unpack regression

Note that $(CONFIG_FS_LITTLEFS_VERSION).tar.gz is expanded to
eg. "v2.5.1".tar.gz. The extra quotes break make's file existence
checks.

Fixes: apache#15148

The regression was caused by apache#14903

Author: yamt

fs/littlefs/Make.defs

@@ -59,7 +59,7 @@ ifeq ($(CONFIG_FS_LITTLEFS_MULTI_VERSION),y)
 CFLAGS += -DLFS_MULTIVERSION
 endif
 
-LITTLEFS_TARBALL = $(CONFIG_FS_LITTLEFS_VERSION).tar.gz
+LITTLEFS_TARBALL = $(shell echo $(CONFIG_FS_LITTLEFS_VERSION).tar.gz)
 
 $(LITTLEFS_TARBALL):
 	$(call DOWNLOAD,https://github.com/ARMmbed/littlefs/archive,$(LITTLEFS_TARBALL),littlefs/$(LITTLEFS_TARBALL))