Update BUILDING

Specify path for additionally headers

Resolves
```- compiling noit_check_log_helpers.c
In file included from noit_check_log_helpers.c:41:0:
bundle.pb-c.h:6:42: fatal error: google/protobuf-c/protobuf-c.h: No such file or directory
 #include <google/protobuf-c/protobuf-c.h>```
Merge pull request #197 from sevan/patch-3

Update BUILDING