Enable thread-safety features in libdbus

There are multiple SDL APIs that internally sink into dbus calls, e.g. battery
status, thread priority. If those calls happen in different threads simultaneously
it can result in dbus crashes.

To abide by dbus's multithreading guidelines we must call dbus_threads_init_default()
to enable dbus's internal locking mechanisms:
https://dbus.freedesktop.org/doc/api/html/group__DBusThreads.html#gac7b8a7001befc3eaa8c6b043151008dc

Additionally, access to a DBusMessage must be synchronized between threads.
SDL was already abiding that guideline as the DBusMessage structs aren't shared.

The following email from the dbus mailing list hints that arbitrating access to
the DBusConnection on the SDL may also be required:
https://lists.freedesktop.org/archives/dbus/2017-September/017306.html

src/core/linux/SDL_dbus.c

@@ -65,6 +65,7 @@ LoadDBUSSyms(void)
     SDL_DBUS_SYM(message_iter_get_arg_type);
     SDL_DBUS_SYM(message_iter_recurse);
     SDL_DBUS_SYM(message_unref);
+    SDL_DBUS_SYM(threads_init_default);
     SDL_DBUS_SYM(error_init);
     SDL_DBUS_SYM(error_is_set);
     SDL_DBUS_SYM(error_free);
@@ -124,6 +125,11 @@ SDL_DBus_Init(void)
             return;  /* oh well */
         }
 
+        if (!dbus.threads_init_default()) {
+            is_dbus_available = SDL_FALSE;
+            return;
+        }
+
         dbus.error_init(&err);
         /* session bus is required */
 

src/core/linux/SDL_dbus.h

@@ -62,6 +62,7 @@ typedef struct SDL_DBusContext {
     int (*message_iter_get_arg_type)(DBusMessageIter *);
     void (*message_iter_recurse)(DBusMessageIter *, DBusMessageIter *); 
     void (*message_unref)(DBusMessage *);
+    dbus_bool_t (*threads_init_default)(void);
     void (*error_init)(DBusError *);
     dbus_bool_t (*error_is_set)(const DBusError *);
     void (*error_free)(DBusError *);