Compatibility with C99 standard

This commit makes the code (mostly) C99-compatible, enabling to compile
it without the -Wno-error=implicit-function-declaration flag. This
way, Magic becomes usable on arm64 architectures, specifically on Apple
computers with M1/M2 SoC.

plot/plotHP.c

@@ -15,6 +15,9 @@
 #include "utils/malloc.h"
 #include "plot/plotInt.h"
 
+/* C99 compat */
+#include "textio/textio.h"
+
 #ifdef VERSATEC
 
 extern int PlotRTLCompress();