Add an entry for X11 "/" key for Brazilian keyboard.

SDL2 reports the following message when we type the "/" on br-abnt2 keyboards:

The key you just pressed is not recognized by SDL. \
To help get this fixed, please report this to the SDL mailing list \
<sdl@libsdl.org> X11 KeyCode 97 (89), X11 KeySym 0x2F (slash).

That's because the corresponding entry in the scancodes table is
marked with value SDL_SCANCODE_UNKNOWN.

This commit fixes that adding the value SDL_SCANCODE_SLASH for this entry.

src/events/scancodes_xfree86.h

@@ -266,7 +266,7 @@ static const SDL_Scancode xfree86_scancode_table2[] = {
     /*  86 */   SDL_SCANCODE_NONUSBACKSLASH,
     /*  87 */   SDL_SCANCODE_F11,
     /*  88 */   SDL_SCANCODE_F12,
-    /*  89 */   SDL_SCANCODE_UNKNOWN,
+    /*  89 */   SDL_SCANCODE_SLASH,
     /*  90 */   SDL_SCANCODE_UNKNOWN,   /* Katakana */
     /*  91 */   SDL_SCANCODE_UNKNOWN,   /* Hiragana */
     /*  92 */   SDL_SCANCODE_UNKNOWN,   /* Henkan_Mode */