Making fetch-dist get the sha256sum files

Fetching these files is part of the release process. When the new
file type was added this step was missed. It will cause the sign
make target to fail.

Signed-off-by: Matt Farina <matt@mattfarina.com>

Makefile

@@ -1,7 +1,7 @@
 BINDIR      := $(CURDIR)/bin
 DIST_DIRS   := find * -type d -exec
 TARGETS     := darwin/amd64 linux/amd64 linux/386 linux/arm linux/arm64 linux/ppc64le linux/s390x windows/amd64
-TARGET_OBJS ?= darwin-amd64.tar.gz darwin-amd64.tar.gz.sha256 linux-amd64.tar.gz linux-amd64.tar.gz.sha256 linux-386.tar.gz linux-386.tar.gz.sha256 linux-arm.tar.gz linux-arm.tar.gz.sha256 linux-arm64.tar.gz linux-arm64.tar.gz.sha256 linux-ppc64le.tar.gz linux-ppc64le.tar.gz.sha256 linux-s390x.tar.gz linux-s390x.tar.gz.sha256 windows-amd64.zip windows-amd64.zip.sha256
+TARGET_OBJS ?= darwin-amd64.tar.gz darwin-amd64.tar.gz.sha256 darwin-amd64.tar.gz.sha256sum linux-amd64.tar.gz linux-amd64.tar.gz.sha256 linux-amd64.tar.gz.sha256sum linux-386.tar.gz linux-386.tar.gz.sha256 linux-386.tar.gz.sha256sum linux-arm.tar.gz linux-arm.tar.gz.sha256 linux-arm.tar.gz.sha256sum linux-arm64.tar.gz linux-arm64.tar.gz.sha256 linux-arm64.tar.gz.sha256sum linux-ppc64le.tar.gz linux-ppc64le.tar.gz.sha256 linux-ppc64le.tar.gz.sha256sum linux-s390x.tar.gz linux-s390x.tar.gz.sha256 linux-s390x.tar.gz.sha256sum windows-amd64.zip windows-amd64.zip.sha256 windows-amd64.zip.sha256sum
 BINNAME     ?= helm
 
 GOPATH        = $(shell go env GOPATH)