add missing common.h file for 25-signal example (#108)

src/25-signal/common.h

@@ -0,0 +1,14 @@
+// SPDX-License-Identifier: BSD-3-Clause
+#ifndef BAD_BPF_COMMON_H
+#define BAD_BPF_COMMON_H
+
+// Simple message structure to get events from eBPF Programs
+// in the kernel to user spcae
+#define TASK_COMM_LEN 16
+struct event {
+    int pid;
+    char comm[TASK_COMM_LEN];
+    bool success;
+};
+
+#endif  // BAD_BPF_COMMON_H