libsdl: add missing undefined ref of "main" to ABI

This symbol is needed to inform the (static) linker about the dependency
of the component-startup code from the program's main function. Without
the symbol, the use of gc-sections during linking may discard the actual
application code.

lib/symbols/sdl

@@ -348,3 +348,4 @@ SDL_timer_running B 4
 SDL_timer_started B 4
 SDL_ultoa T
 _Z15sdl_init_genodeRN6Genode3EnvE T
+main U