Linux go mod vendor 后，编译出错

go mod vendor 后，编译出错，提示无法找到 webp.h 文件

```shell
# cat /etc/redhat-release
CentOS Linux release 7.9.2009 (Core)

# go version
go version go1.15.5 linux/amd64

# go build
# github.com/chai2010/webp
vendor/github.com/chai2010/webp/capi.go:24:18: fatal error: webp.h: No such file or directory
#include "webp.h"
^
compilation terminated.
```

Reproduce the failure on the reported CentOS 7.9 and Go 1.15.5 setup with go mod vendor and go build. Inspect vendor/github.com/chai2010/webp/capi.go and its webp.h include to determine the missing build prerequisite. Done means the reported build failure is resolved or the required prerequisite is clearly documented.

Written by the indexing model from the issue text.