Only recognise spaceship operator for C++

I don't think any valid C program can contain `<=>` in a tokenisable
context, but it's more helpful to fail with a syntax error at SWIG
parse time and not potentially generate C code trying to use `<=>` in
an expression which then fails at compile time.

Source/Swig/scanner.c

@@ -896,7 +896,7 @@ static int look(Scanner *s) {
       else if (c == '=') {
 	if ((c = nextchar(s)) == 0) {
 	  return SWIG_TOKEN_LTEQUAL;
-	} else if (c == '>') { /* Spaceship operator */
+	} else if (c == '>' && cparse_cplusplus) { /* Spaceship operator */
 	  return SWIG_TOKEN_LTEQUALGT;
 	} else {
 	  retract(s, 1);