Fix exponentiation with immutable matrix

[jishnub]

The following works after this:

julia> using LinearAlgebra, StaticArrays

julia> S = @SMatrix [1 2; 3 4]
2×2 SMatrix{2, 2, Int64, 4} with indices SOneTo(2)×SOneTo(2):
 1  2
 3  4

julia> 2^S
2×2 SMatrix{2, 2, Float64, 4} with indices SOneTo(2)×SOneTo(2):
 10.4827  14.1519
 21.2278  31.7106

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.03%. Comparing base (830ea2f) to head (e068ea3).
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1289   +/-   ##
=======================================
  Coverage   92.03%   92.03%           
=======================================
  Files          34       34           
  Lines       15499    15499           
=======================================
  Hits        14265    14265           
  Misses       1234     1234           

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