Declare loadModelWithProgress in jllama.h to fix UnsatisfiedLinkError

PR #188 added loadModelWithProgress in Java (LlamaModel.java:380) and a
matching JNIEXPORT definition in jllama.cpp (line 747), but forgot to
add the forward declaration to the legacy hand-rolled jllama.h. The
generated header net_ladenthin_llama_LlamaModel.h has it, but jllama.cpp
only #includes jllama.h. Without the prior 'extern "C"' declaration in
scope, the C++ compiler treats the function definition as a normal C++
function and produces a mangled symbol:

  _Z57Java_net_ladenthin_llama_LlamaModel_loadModelWithProgress...

while the JVM looks up the unmangled name and throws

  UnsatisfiedLinkError: 'void net.ladenthin.llama.LlamaModel
    .loadModelWithProgress(java.lang.String[],
                           net.ladenthin.llama.LoadProgressCallback)'

at runtime. Adding the declaration to jllama.h restores C linkage. The
plain loadModel symbol was unaffected because its declaration was
already present.

Verified locally with nm -D libjllama.so:
  before: _Z57Java_..._loadModelWithProgress... (mangled)
  after:  Java_..._loadModelWithProgress (plain C)

This unblocks LoadProgressCallbackTest on all platforms.

Author: claude

src/main/cpp/jllama.h

@@ -70,6 +70,13 @@ JNIEXPORT jbyteArray JNICALL Java_net_ladenthin_llama_LlamaModel_decodeBytes(JNI
  */
 JNIEXPORT void JNICALL Java_net_ladenthin_llama_LlamaModel_loadModel(JNIEnv *, jobject, jobjectArray);
 
+/*
+ * Class:     net_ladenthin_llama_LlamaModel
+ * Method:    loadModelWithProgress
+ * Signature: ([Ljava/lang/String;Lnet/ladenthin/llama/LoadProgressCallback;)V
+ */
+JNIEXPORT void JNICALL Java_net_ladenthin_llama_LlamaModel_loadModelWithProgress(JNIEnv *, jobject, jobjectArray, jobject);
+
 /*
  * Class:     net_ladenthin_llama_LlamaModel
  * Method:    delete