Add jemalloc placeholder on Windows

Due to the way how `generated_extension_loader_package_build.cpp` is
generated in the latest `main` versions of the engine, it expects to
have the same set of linked extensions on all platforms. `jemalloc`
extension is not available on Windows, so the JDBC build fails on
Windows.

This PR adds an emtpy placeholder for `JemallocExtension` class that is
expected by `generated_extension_loader_package_build.cpp` to fix the
compilation on Windows.

Author: staticlibs

CMakeLists.txt

@@ -569,6 +569,7 @@ add_jar(duckdb_jdbc_tests ${JAVA_TEST_FILES} INCLUDE_JARS duckdb_jdbc_nolib)
 # main shared lib compilation
 
 if(MSVC)
+  list(APPEND DUCKDB_INCLUDE_DIRS src/windows)
   list(APPEND DUCKDB_SRC_FILES duckdb_java.def)
 else()
   list(APPEND DUCKDB_SRC_FILES ${JEMALLOC_SRC_FILES})

CMakeLists.txt.in

@@ -95,6 +95,7 @@ add_jar(duckdb_jdbc_tests ${JAVA_TEST_FILES} INCLUDE_JARS duckdb_jdbc_nolib)
 # main shared lib compilation
 
 if(MSVC)
+  list(APPEND DUCKDB_INCLUDE_DIRS src/windows)
   list(APPEND DUCKDB_SRC_FILES duckdb_java.def)
 else()
   list(APPEND DUCKDB_SRC_FILES ${JEMALLOC_SRC_FILES})

src/windows/jemalloc_extension.hpp

@@ -0,0 +1,18 @@
+#pragma once
+
+#include "duckdb.hpp"
+
+namespace duckdb {
+
+class JemallocExtension : public Extension {
+public:
+	void Load(ExtensionLoader &) override {};
+	std::string Name() override {
+		return "";
+	};
+	std::string Version() const override {
+		return "";
+	};
+};
+
+} // namespace duckdb