llama: fix GGML_METAL=ON build - missing ggml-metal.h include in llama-dflash.cpp (#2134)

llama-dflash.cpp calls ggml_backend_is_metal() and
ggml_backend_metal_set_n_cb() inside an #ifdef GGML_USE_METAL block but
never includes ggml-metal.h, so any Metal-enabled build fails to
compile. Add the same guarded include llama.cpp already uses.

src/llama-dflash.cpp

@@ -9,6 +9,10 @@
 #include "ggml.h"
 #include "ggml-backend.h"
 
+#ifdef GGML_USE_METAL
+#  include "ggml-metal.h"
+#endif
+
 #include <algorithm>
 #include <cmath>
 #include <cstring>