user supplied header file accidentally named 'wgtcc.h'

create a header file named `wgtcc.h`:
```
fuck
```

`foo.c`:
```
#include <wgtcc.h>
void foo() {}
```

```
$ wgtcc -E foo.c -I.

fuckfuck
void foo() {}
```

The output is strange.

If we don't create `wgtcc.h`, the internal `wgtcc.h` would be included accidentally.

In short, the internal `wgtcc.h` must _NOT_ be placed in the standard including path.