plugins: define plugin API symbols as extern "C" when compiling in C++

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/qemu-devel/20260124182921.531562-5-pierrick.bouvier@linaro.org
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>

include/qemu/qemu-plugin.h

@@ -16,6 +16,10 @@
 #include <stdbool.h>
 #include <stddef.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /*
  * For best performance, build the plugin with -fvisibility=hidden so that
  * QEMU_PLUGIN_LOCAL is implicit. Then, just mark qemu_plugin_install with
@@ -1245,4 +1249,8 @@ void qemu_plugin_u64_set(qemu_plugin_u64 entry, unsigned int vcpu_index,
 QEMU_PLUGIN_API
 uint64_t qemu_plugin_u64_sum(qemu_plugin_u64 entry);
 
+#ifdef __cplusplus
+} /* extern "C" */
+#endif
+
 #endif /* QEMU_QEMU_PLUGIN_H */