cogl: fix build with gcc15

- add "-std=gnu17" to `env.NIX_CFLAGS_COMPILE`

Upstream repo is archived:
https://gitlab.gnome.org/Archive/cogl

Other distros also used "-std=gnu17":
https://gitlab.alpinelinux.org/alpine/aports/-/commit/d9e057053f20fcc7d46c7f8c6222100a88e017e8
https://github.com/gentoo/gentoo/commit/379ca097b966b40890b5a4572502b8f9352a374d
https://src.fedoraproject.org/rpms/cogl/c/442af4e8c3e17e8d5b051a4e1600f23f366f5554

Fixes build failure with gcc15:
```
cogl-path.c:1361:20: error: passing argument 3 of 'gluTessCallback'
from incompatible pointer type
[8;;https://gcc.gnu.org/onlinedocs/gcc-15.2.0/gcc/Warning-Options.html#index-Wincompatible-pointer-types-Wincompatible-pointer-types8;;]
 1361 |                    _cogl_path_tesselator_begin);
      |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                    |
      |                    void (*)(GLenum,  CoglPathTesselator *) {aka void (*)(unsigned int,  struct _CoglPathTesselator *)}
In file included from cogl-path.c:49:
tesselator/tesselator.h:57:70: note: expected 'void (*)(void)' but
argument is of type 'void (*)(GLenum,  CoglPathTesselator *)'
{aka 'void (*)(unsigned int,  struct _CoglPathTesselator *)'}
   57 | void gluTessCallback (GLUtesselator* tess, GLenum which, _GLUfuncptr CallBackFunc);
      |                                                          ~~~~~~~~~~~~^~~~~~~~~~~~
cogl-path.c:1094:1: note: '_cogl_path_tesselator_begin' declared here
 1094 | _cogl_path_tesselator_begin (GLenum type,
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~

```

0 files changed, 0 insertions, 0 deletions