Correct ibus include path

ibus-1.0.pc adds /usr/include/ibus-1.0 to the search path but not
/usr/include, which makes the canonical include path <ibus.h>.

src/core/linux/SDL_ibus.h

@@ -26,7 +26,7 @@
 
 #ifdef HAVE_IBUS_IBUS_H
 #define SDL_USE_IBUS 1
-#include <ibus-1.0/ibus.h>
+#include <ibus.h>
 
 extern SDL_bool SDL_IBus_Init(void);
 extern void SDL_IBus_Quit(void);