Add dependent system includes to make sudo_*.h more standalone.

In the past we've relied on the various .c files to include the
system headers that define types that the sudo_*.h headers require.
This is fragile and can cause issues when includes get re-ordered.

include/sudo_conf.h

@@ -19,6 +19,12 @@
 #ifndef SUDO_CONF_H
 #define SUDO_CONF_H
 
+#ifdef HAVE_STDBOOL_H
+# include <stdbool.h>
+#else
+# include "compat/stdbool.h"
+#endif
+
 #include "sudo_queue.h"
 
 /* Flags for sudo_conf_read() */