Use angle brackets when including headers that can only be found when

an -I flag is specified.  The files in the compat dir could get away
with double quotes here but I've converted all the source files
to use angle brackets for consistency.

plugins/sample/sample_plugin.c

@@ -49,8 +49,8 @@
 #include <stdarg.h>
 
 #include <sudo_plugin.h>
-#include "compat.h"
-#include "missing.h"
+#include <compat.h>
+#include <missing.h>
 
 /*
  * Sample plugin module that allows any user who knows the password