src/SDL.c: add managarm support

src/SDL.c

@@ -609,6 +609,8 @@ const char *SDL_GetPlatform(void)
     return "Nokia N-Gage";
 #elif defined(__3DS__)
     return "Nintendo 3DS";
+#elif defined(__managarm__)
+    return "Managarm";
 #else
     return "Unknown (see SDL_platform.h)";
 #endif