fix nginx compile error unable to include common/AgentStarter.h

this is not very pretty, but maybe it's a help for someone else running into the same issue...

```
==> ./configure --prefix=/opt/boxen/homebrew/Cellar/nginx/1.4.4-boxen1 --with-http_ssl_module --with-pcre --with-ipv6 --with-cc-opt='-I/opt/boxen/homebrew/include' --with-ld-opt='-L/opt/boxen/homebrew/lib' --conf-path=/opt/boxen/config/nginx/nginx.conf --pid-path=/opt/boxen/data/nginx/nginx.pid --lock-path=/opt/boxen/data/nginx/nginx.lock --add-module=/opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/passenger-4.0.27/ext/nginx
==> make
#include "common/AgentsStarter.h"
         ^
1 error generated.
make[1]: *** [objs/addon/nginx/ngx_http_passenger_module.o] Error 1
make: *** [build] Error 2
```

I tried adding
` -I/opt/boxen/rbenv/versions/2.0.0/lib/ruby/gems/2.0.0/gems/passenger-4.0.27/ext` (folder of common/AgentsStarter.h) to the build options but that did not help ...