[rust bmp] Add enum name to Skcms_Signature enum

To be able to use the enum defining Skcms_Signature values from rust's
cxx bridge it requires to be named. This change is a pre-requisite for
this CL:

https://skia-review.googlesource.com/c/skia/+/1106557

Bug: chromium:452666425
Change-Id: I3f15a37253e4c17c5a542efd51230f9ed2239f93
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/1111536
Reviewed-by: Łukasz Anforowicz <lukasza@google.com>
Commit-Queue: Łukasz Anforowicz <lukasza@google.com>
Reviewed-by: Kaylee Lubick <kjlubick@google.com>