Add inline to free function in header

[elshize]

Must be inline or else fails when linking two translation units that both include it.

[codecov]

Codecov Report

Merging #466 (5fde3c5) into master (a28100d) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #466      +/-   ##
==========================================
- Coverage   93.26%   93.23%   -0.03%     
==========================================
  Files          92       92              
  Lines        4926     4926              
==========================================
- Hits         4594     4593       -1     
- Misses        332      333       +1     

Impacted Files	Coverage Δ
include/pisa/wand_data.hpp	85.55% <100.00%> (ø)
include/pisa/bit_vector.hpp	97.54% <0.00%> (-0.41%)	⬇️

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