Fix compilation with LLVM 20

[victoryforce]

Currently, compiling darktable code with LLVM 20 fails with the following error:

In file included from C:/msys64/home/Victor/darktable/src/common/exif.cc:42:
In file included from C:/msys64/home/Victor/darktable/src/control/control.h:32:
In file included from C:/msys64/home/Victor/darktable/src/control/jobs.h:24:
In file included from C:/msys64/home/Victor/darktable/src/views/view.h:21:
In file included from C:/msys64/home/Victor/darktable/src/common/act_on.h:21:
C:/msys64/home/Victor/darktable/src/gui/gtk.h:407:104: fatal error: missing sentinel in function call [-Wsentinel]
  407 |   g_object_set(label, "halign", GTK_ALIGN_START, "xalign", 0.0f, "ellipsize", PANGO_ELLIPSIZE_END, NULL);
      |                                                                                                        ^
      |                                                                                                        , nullptr

We can't use NULL here. The problem is that in this case the code ends up being included from a C++ source file (exif.cc), and in C++ (unlike plain C) NULL is defined as simply 0 or 0LL (which is an integer constant, but not a pointer type), while in plain C it is defined as ((void *)0).

LLVM 20 starts flagging this as a warning (which becomes an error in our build process), forcing sentinel to be a null pointer, not just a null value.

The need for sentinel to be a pointer type is documented in https://gcc.gnu.org/onlinedocs/gcc-14.1.0/gcc/Common-Function-Attributes.html#index-sentinel-function-attribute:

A valid NULL in this context is defined as zero with any pointer type. If your system defines the NULL macro with an integer type then you need to add an explicit cast.

[TurboGit]

Thanks!

(maybe you could have kept NULL : (void *)NULL).

[victoryforce]

(maybe you could have kept NULL : (void *)NULL).

Then readers of the code who know the definition of NULL in C will wonder why this is necessary. After all, in all other cases (when the code is included from the plain C source files), "(void *)NULL" will expand to "(void *)(void *)0".

I'd better try to write a compact explanation and add a comment with it. Just to document why our code has to look like this, departing from the established pattern.