[sentry-native] Fix build failure and cmake path

[JackBoosY]

When building sentry-native on Windows, the compiler sometimes reports some errors:

D:\buildtrees\sentry-native\src\try-native-a1d92af944.clean\src\../vendor/mpack.h(2113): warning C4013: '_Static_assert' undefined; assuming extern returning int
mpack.c.obj : error LNK2019: unresolved external symbol _Static_assert referenced in function mpack_writer_init_growable

This is because MPACK_STATIC_ASSERT is not correctly defined as static_assert.
Fix this issue and fix the generated cmake files path.

Related: #12612.

[JackBoosY]

Perhaps some ports polluted the definition of the macro MPACK_STATIC_ASSERT, which caused the issue.
If so, this modification will be invalid.

[BillyONeal]

Maybe we should do something like BillyONeal@4f042be instead?

[BillyONeal]

Thanks for your contribution!