webgpu.h: Fix WGPU_WHOLE_SIZE causing linking errors.

[Kangz]

The symbol was not marked as static and would cause linking errors
because it would be defined in multiple translation units. Replace it
with a more traditional C-style #define.

Found by @floooh.

[kvark]

would adding static make it stronger instead?

[floooh]

static worked too when I tested, but since this is specifically a fix for the C language, IMHO a #define is the better choice - it's usually only the C++ people who have a problem with the C preprocessor ;)

[Kangz]

@kvark given the above, WDYT of using the #define?

[kvark]

I don't have a strong preference here, we can proceed :)
Perhaps, somewhere close to MVP we can call some people who know what idiomatic C is, to review the header API.

[Kangz]

Thanks for the review!