src/util.h: switched file_exists macro to function.

the macro returns true for directories aswell, using stat.h instead.

Makefile

@@ -33,6 +33,7 @@ obj += src/path.o
 obj += src/strbuf.o
 obj += src/xalloc.o
 obj += src/die.o
+obj += src/file.o
 
 obj += src/inotify.o
 obj += src/event.o