Fix a few strict compiler warnings.

tests/example.c

@@ -24,6 +24,8 @@
 
 
 int main(int argc, char **argv) {
+    (void)argv;
+
     fprintf(stderr, "write to stderr: %d\n", argc);
     printf("write to stdout\n");
     fflush(stdout);

tests/example_vfork.c

@@ -19,6 +19,7 @@
 
 #include <stdio.h>
 #include <stdlib.h>
+#include <sys/types.h>
 #include <unistd.h>
 
 int main(int argc, char **argv) {