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.

Examples/test-suite/errors/c_spaceship.i

@@ -0,0 +1,3 @@
+%module xxx
+
+int a[(1<=>2>1)];

Examples/test-suite/errors/c_spaceship.stderr

@@ -0,0 +1 @@
+c_spaceship.i:3: Error: Syntax error in input(1).