build: remove @ as it's causing an error

Since $(DownloadMethod/unknown) is being invoked in the expansion of
$(call locked ...) anyway, you can't have an @ because the shell
doesn't know what to do with it.

Signed-off-by: Philip Prindeville <[email protected]>

include/download.mk

@@ -103,7 +103,7 @@ hash_var = $(if $(filter-out x,$(1)),MD5SUM,HASH)
 endif
 
 define DownloadMethod/unknown
-	@echo "ERROR: No download method available"; false
+	echo "ERROR: No download method available"; false
 endef
 
 define DownloadMethod/default