avdevice/sdl2: Suppress macro redefinition warning

SDL2's headers #define HAVE_PTHREAD_SETNAME_NP (to one here);
we also have a config define which leads the compiler to
warn about this when the values are distinct (as they are here).
Suppress this warning by including config.h before SDL.h.
The latter will then redefine our define which doesn't matter
and which does not lead to warnings because compilers don't warn
about system headers by default (depending upon -Wsystem-headers).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavdevice/sdl2.c

@@ -23,6 +23,8 @@
  * libSDL2 output device
  */
 
+#include "config.h"
+
 #include <SDL.h>
 #include <SDL_thread.h>