Tools: properly handle the keyboard interrupt (#2083)

Many tools rely on the user to type Ctrl-C to end, but don't actually
catch the keyboard interrupt and thus show an ugly backtrace when it
happens. Let's catch the interrupt.
diff --git a/tools/inject.py b/tools/inject.py
index 741a0fb..bf07a8f 100755
--- a/tools/inject.py
+++ b/tools/inject.py
@@ -507,7 +507,10 @@
 
     def _main_loop(self):
         while True:
-            self.bpf.perf_buffer_poll()
+            try:
+                self.bpf.perf_buffer_poll()
+            except KeyboardInterrupt:
+                exit()
 
     def run(self):
         self._create_probes()