cgen: fix for/in codegen when iterating over C structs

[spiveeworks]

When implementing next for a C struct, and then looping over it with for/in, it was generating MyCStruct_next(&_t) or even struct MyCStruct_next(&_t) instead of C__MyCStruct_next(&_t). See issue #21047 for more details.

[spytheman]

It can be tested similarly to https://github.com/vlang/v/pull/21042/files, i.e.
0) create vlib/v/tests/c_structs/v.mod (it can be empty text file).

1. create vlib/v/tests/c_structs/iterator.c:

struct MyCStruct {
        int x;
};

2. create vlib/v/tests/c_structs/cstruct_iterator_test.c.v:

#include "@VMODROOT/iterator.c"

struct C.MyCStruct {
mut:
	x int
}

fn (mut self C.MyCStruct) next() ?int {
	if self.x >= 10 {
		return none
	}
	self.x++
	return self.x
}

fn test_iterating_over_cstructs() {
	iter := C.MyCStruct{}
	for x in iter {
		println(x)
		assert true
	}
}