tests: Update run-make/issue-25581 to reflect how fat pointers are passed.

src/test/run-make/issue-25581/test.c

@@ -2,15 +2,10 @@
 #include <stddef.h>
 #include <stdint.h>
 
-struct ByteSlice {
-        uint8_t *data;
-        size_t len;
-};
-
-size_t slice_len(struct ByteSlice bs) {
-        return bs.len;
+size_t slice_len(uint8_t *data, size_t len) {
+    return len;
 }
 
-uint8_t slice_elem(struct ByteSlice bs, size_t idx) {
-        return bs.data[idx];
+uint8_t slice_elem(uint8_t *data, size_t len, size_t idx) {
+    return data[idx];
 }