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/plotCmd.c

@@ -41,6 +41,9 @@ static char rcsid[] __attribute__ ((unused)) = "$Header: /usr/cvsroot/magic-8.0/
 #include "commands/commands.h"
 #include "plot/plotInt.h"
 
+/* C99 compat */
+#include "textio/textio.h"
+
 
 /*
  * ----------------------------------------------------------------------------