rp2040_flash: Fix the type when printing num_blocks (#6279)

image->num_blocks is of type unsigned size_t thus unsigned int
or unsigned long int.

%lu specifies a type of long unsigned int. Thus resulting in compiler warning about type mismatch on some systems.
Fix this by printing the value with length modified z.

Signed-off-by: Martin Botka <martin.botka@somainline.org>

lib/rp2040_flash/main.c

@@ -174,7 +174,7 @@ int main(int argc, char *argv[]) {
         rc = 1;
         goto do_exit;
     }
-    fprintf(stderr, "Loaded UF2 image with %lu pages\n", image->num_blocks);
+    fprintf(stderr, "Loaded UF2 image with %zu pages\n", image->num_blocks);
 
     bool has_target = false;
     uint8_t target_bus = 0;