Beware of using random online compilers as there is a chance that they can build in malicious code (like a back door). Always compile plugins locally if at all possible.
Also, HIT_STOMATCH is not defined anywhere in AMX Mod X. However, HIT_STOMACH is defined in amxmisc.inc (notice that the real constant only has one T and not two). If it is truly spelled wrong in one of your includes then it needs to be resolved either by defining the term or if it is supposed to be HIT_STOMACH then it needs corrected. Since this error is in your file, I'd actually consider double checking that you have the correct include file because it's possible that there are other issues in this one.