Add generic SDL_syscond to N3DS threads source list

CMakeLists.txt

@@ -2887,6 +2887,7 @@ elseif(N3DS)
   set(SDL_THREAD_N3DS 1)
   sdl_glob_sources("${SDL3_SOURCE_DIR}/src/thread/n3ds/*.c")
   sdl_sources(
+    "${SDL3_SOURCE_DIR}/src/thread/generic/SDL_syscond.c"
     "${SDL3_SOURCE_DIR}/src/thread/generic/SDL_systls.c"
     "${SDL3_SOURCE_DIR}/src/thread/generic/SDL_sysrwlock.c"
   )