Prefix for glob mode should be equal abs dir of bindata.go.

Referring to example https://github.com/rjeczalik/bindata/#automagic-conversion-within-gopath-workspace for
```
/home/user/src/github.com/user/example/bindata.go
```
 this will change map key from:
```
github.com/user/example/assets/css/default.css
```
to
```
assets/css/default.css
```