Print "OK!" in example for <kernaux/printf.h>

examples/printf.c

@@ -1,6 +1,7 @@
 #include <kernaux/printf.h>
 
 #include <assert.h>
+#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 
@@ -22,5 +23,7 @@ int main()
     kernaux_printf(my_putchar, "Hello, %s!", "Alex");
     assert(strcmp(buffer, "Hello, Alex!") == 0);
 
+    printf("OK!\n");
+
     return 0;
 }