mac-virtualcam: Fix DAL plugin entrypoint not being exported

With the recent change to enforce C17 standard for compilers, default
visibility was also enforced to be hidden. This also automatically hid
the `PlugInMain` symbol required for DAL plugins. Adding the
`__exported` decorator makes the symbol explicitly visible.

plugins/mac-virtualcam/src/dal-plugin/OBSDALPlugInMain.mm

@@ -23,7 +23,7 @@
 
 //! PlugInMain is the entrypoint for the plugin
 extern "C" {
-void *PlugInMain(CFAllocatorRef, CFUUIDRef requestedTypeUUID)
+__exported void *PlugInMain(CFAllocatorRef, CFUUIDRef requestedTypeUUID)
 {
 	DLogFunc(@"version=%@", PLUGIN_VERSION);
 	if (!CFEqual(requestedTypeUUID, kCMIOHardwarePlugInTypeID)) {