Fix clang build failure

src/include/86box/plat.h

@@ -55,6 +55,10 @@ extern int strnicmp(const char *s1, const char *s2, size_t n);
 #    define off64_t  off_t
 #endif
 
+#if !defined (__APPLE__) || !defined(__clang__)
+#    define FALLTHROUGH_ANNOTATION
+#endif
+
 #ifdef _MSC_VER
 #    define UNUSED(arg) arg
 #else

src/include/86box/plat_fallthrough.h

@@ -0,0 +1,25 @@
+/*
+ * 86Box    A hypervisor and IBM PC system emulator that specializes in
+ *          running old operating systems and software designed for IBM
+ *          PC systems and compatibles from 1981 through fairly recent
+ *          system designs based on the PCI bus.
+ *
+ *          This file is part of the 86Box distribution.
+ *
+ *          Define the various platform support functions.
+ *
+ *
+ *
+ * Authors: Miran Grca, <jasmine@iwanek.co.uk>
+ *
+ *          Copyright 2023 Jasmine Iwanek
+ */
+
+#ifndef EMU_PLAT_FALLTHROUGH_H
+#define EMU_PLAT_FALLTHROUGH_H
+
+#if !defined (__APPLE__) || !defined(__clang__)
+#    define FALLTHROUGH_ANNOTATION
+#endif
+
+#endif /*EMU_PLAT_FALLTHROUGH_H*/