Add missing header to fix an s390x specific compile problem

Add missing header to fix an s390x specific compile problem

Add include unistd.h because of the close() call.
The error message on s390x/rhel8 was:

-------------------8<----------------------------------------------------
tapset-debuginfod.cxx:44:3: error: 'close' was not declared in this scope
   close(metadata_fd);
   ^~~~~
tapset-debuginfod.cxx:44:3: note: suggested alternative: 'pclose'
   close(metadata_fd);
   ^~~~~
   pclose
make[3]: *** [Makefile:1756: stap-tapset-debuginfod.o] Error 1
make[3]: *** Waiting for unfinished jobs....
-------------------8<----------------------------------------------------