`syntaxError` in include is hidden - leading to suspected false negative

`test.h`
```cpp
enum Type
{
ERROR
};

struct A
{
};
```

`test.cpp`
```cpp
#include "test.h"

extern void f()
{
A a; // unusedVariable
}
```

In Cppcheck 2.4 the header will generate a `syntaxError` with the Windows version:
```
test.h:1:1: error: syntax error [syntaxError]
enum Type
^
```
This leads to the `unusedVariable` in function `f()` not being reported making it seems like a false negative since the `syntaxError` is being suppressed as it didn't occur in the file we are analyzing.

We need to make this error visible - possibly via a global warning about this.

There's most likely other errors which may lead to the same problem.