fix a memory leak for getline()

The memory allocated by getline() is not freed.
This may cause some tool like clang memory
leak sanitizer complain.
Signed-off-by: Yonghong Song <yhs@fb.com>

src/cc/libbpf.c

@@ -1014,6 +1014,7 @@ static int bpf_detach_probe(const char *ev_name, const char *event_type)
       found_event = 1;
       break;
     }
+  free(cptr);
   fclose(fp);
   fp = NULL;