Cross compiling to windows using clang trips static asserts

```
$ clang nuklear.h -target x86_64-windows-gnu
nuklear.h:435:1: error: '_dummy_array435' declared as an array with a negative size
NK_STATIC_ASSERT(sizeof(nk_size) >= sizeof(void*));
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
nuklear.h:293:75: note: expanded from macro 'NK_STATIC_ASSERT'
#define NK_STATIC_ASSERT(exp) typedef char NK_UNIQUE_NAME(_dummy_array)[(exp)?1:-1]
^~~~~~~~~~
nuklear.h:436:1: error: '_dummy_array436' declared as an array with a negative size
NK_STATIC_ASSERT(sizeof(nk_ptr) >= sizeof(void*));
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
nuklear.h:293:75: note: expanded from macro 'NK_STATIC_ASSERT'
#define NK_STATIC_ASSERT(exp) typedef char NK_UNIQUE_NAME(_dummy_array)[(exp)?1:-1]
^~~~~~~~~~
2 errors generated.

```

Reproduce the failure with the shown clang command, then inspect nk_size, nk_ptr, and the NK_STATIC_ASSERT definition in nuklear.h. Determine why their size assertions fail for the x86_64-windows-gnu target; done means this command compiles without negative-size assertion errors while preserving the checks for other targets.

Written by the indexing model from the issue text.