importC is not working in GDC?

[felixfxu]

Hi,

I'm testing _importC_ and I found that _importC_ works in both 
_dmd_ and _ldc_ but not _gdc_.

In Ubuntu 24.04, I am testing with these files:

```d
//app.d
import std.stdio;
import importctest;
void main()
{
	importctest.test();
}
```
and
```c
//importtest.c
#include <stdio.h>
void test()
{
     printf("importC is available\n");
}
```
and compile them by:
`dmd source/app.d source/importctest.c` OK

`ldc2 source/app.d source/importctest.c` OK,

but with gdc:
>gdc source/app.d source/importctest.c
source/app.d:2:8: error: unable to read module ‘importctest’
     2 | import importctest;
       |        ^
source/app.d:2:8: note: Expected 'importctest.d' or 
'importctest/package.d' in one of the following import paths:
     2 | import importctest;
       |        ^
import path[0] = /usr/lib/gcc/x86_64-linux-gnu/13/include/d