sam : add default case for unsupported prompt types (#1263)

sam : add default case for unsupported prompt types (#1263)

This commit adds a default case in the `embed_prompt_sparse` lambda
function.

The motivation for this change is that currently the following warning
is generated when compiling:
```console
/ggml/examples/sam/sam.cpp: In lambda function:
/ggml/examples/sam/sam.cpp:1499:5: warning:
control reaches end of non-void function [-Wreturn-type]
 1499 |     }();
      |     ^
```