Makefile: Use "! -path './vendor/*'" for GOFILES

This lets us drop the grep, which may give some very slight
performance increase.  But mostly this just reduces the complexity of
a pipe, which we've had for GOFMT since b4f2d7d (Initial asset
rendering & secure api endpoint, 2016-03-21, #2).

Both ! and -path are in POSIX [1].

[1]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/find.html

Author: wking

Makefile

@@ -3,7 +3,7 @@ export GOARCH:=amd64
 export PATH:=$(PATH):$(PWD)
 
 LOCAL_OS:=$(shell uname | tr A-Z a-z)
-GOFILES:=$(shell find . -name '*.go' | grep -v -E '(./vendor)')
+GOFILES:=$(shell find . -name '*.go' ! -path './vendor/*')
 LDFLAGS=-X github.com/kubernetes-incubator/bootkube/pkg/version.Version=$(shell $(CURDIR)/build/git-version.sh)
 TERRAFORM:=$(shell command -v terraform 2> /dev/null)