Removed SDL_bool in favor of plain bool

We require stdbool.h in the build environment, so we might as well use the plain bool type.

If your environment doesn't have stdbool.h, this simple replacement will suffice:
typedef signed char bool;

test/testsurround.c

@@ -87,12 +87,12 @@ get_channel_name(int channel_index, int channel_count)
     case 7:
         return "Side Right";
     }
-    SDLTest_AssertCheck(SDL_FALSE, "Invalid channel_index for channel_count:  channel_count=%d channel_index=%d", channel_count, channel_index);
+    SDLTest_AssertCheck(false, "Invalid channel_index for channel_count:  channel_count=%d channel_index=%d", channel_count, channel_index);
     SDL_assert(0);
     return NULL;
 }
 
-static SDL_bool is_lfe_channel(int channel_index, int channel_count)
+static bool is_lfe_channel(int channel_index, int channel_count)
 {
     return (channel_count == 3 && channel_index == 2) || (channel_count >= 6 && channel_index == 3);
 }