jit: fix build with LLVM-21

LLVM-21 renamed llvm::GlobalValue::getGUID() to
getGUIDAssumingExternalLinkage(), so add a version guard.

Author: Holger Hoffstätte <holger@applied-asynchrony.com>
Discussion: https://www.postgresql.org/message-id/flat/d25e6e4a-d1b4-84d3-2f8a-6c45b975f53d%40applied-asynchrony.com

src/backend/jit/llvm/llvmjit_inline.cpp

@@ -243,7 +243,11 @@ llvm_build_inline_plan(LLVMContextRef lc, llvm::Module *mod)
 
 		llvm_split_symbol_name(symbolName.data(), &cmodname, &cfuncname);
 
+#if LLVM_VERSION_MAJOR >= 21
+		funcGUID = llvm::GlobalValue::getGUIDAssumingExternalLinkage(cfuncname);
+#else
 		funcGUID = llvm::GlobalValue::getGUID(cfuncname);
+#endif
 
 		/* already processed */
 		if (inlineState.processed)