fix(katana): place import inside its feature scope

[kariy]

make sure katana-provider crate is compilable without the default features. currently running cargo check --no-default-features on the crate will return error:

  |
7 | use crate::providers::fork::backend::ForkedBackendError;
  |                       ^^^^ could not find `fork` in `providers`
  |

this change will fix it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.30%. Comparing base (af9cdf6) to head (540e90e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1902   +/-   ##
=======================================
  Coverage   70.29%   70.30%           
=======================================
  Files         315      315           
  Lines       35850    35850           
=======================================
+ Hits        25201    25203    +2     
+ Misses      10649    10647    -2     

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