llama : add static_cast to fix CI warning/error

This commit attempts to fix the following warning/error:

```console
src/llama.cpp:7271:31: error:
comparison of integer expressions of different signedness:
‘int’ and ‘uint32_t’ {aka ‘unsigned int’} [-Werror=sign-compare]
 7271 |                         if (i < hparams.n_layer_dense_lead) {
      |                             ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
```
This can be reproduced locally by setting -Wsign-compare in the
Makefile.

Signed-off-by: Daniel Bevenius <[email protected]>

Author: danbev

src/llama.cpp

@@ -7268,7 +7268,7 @@ static bool llm_load_tensors(
 
                         layer.ffn_norm = ml.create_tensor(ctx_layer, tn(LLM_TENSOR_FFN_NORM, "weight", i), {n_embd});
 
-                        if (i < hparams.n_layer_dense_lead) {
+                        if (static_cast<uint32_t>(i) < hparams.n_layer_dense_lead) {
                             layer.ffn_gate = ml.create_tensor(ctx_split, tn(LLM_TENSOR_FFN_GATE, "weight", i), {n_embd,   n_ff});
                             layer.ffn_down = ml.create_tensor(ctx_split, tn(LLM_TENSOR_FFN_DOWN, "weight", i), {  n_ff, n_embd});
                             layer.ffn_up   = ml.create_tensor(ctx_split, tn(LLM_TENSOR_FFN_UP,   "weight", i), {n_embd,   n_ff});