Just to fill out an old thread with another answer:
If you are doing this in a file that is NOT your main one, then add the same #include to your main file (the one with loop in it). Amazingly the include file suddenly is found in all files. Total guess but I think that the Arduino IDE is trying to be helpful by scanning your sketch and automatically adding -I lines to the compile. But its not doing so for all .c and .h in your sketch so it ends up outsmarting itself and confusing mere mortals like you and I. 