Use relative include paths (NFC)

[karwa]

This allows the project to be built without specifying header search paths, which is necessary to import it as a SwiftPM package.

[codecov-io]

Codecov Report

Merging #104 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #104   +/-   ##
=======================================
  Coverage   82.86%   82.86%           
=======================================
  Files          62       62           
  Lines       12827    12827           
=======================================
  Hits        10629    10629           
  Misses       2198     2198

Impacted Files	Coverage Δ
enc/koi8_r.c	53.84% <ø> (ø)	⬆️
enc/iso_8859_14.c	3.5% <ø> (ø)	⬆️
enc/utf_16le.c	94.82% <ø> (ø)	⬆️
enc/iso_8859_15.c	20.37% <ø> (ø)	⬆️
enc/iso_8859_9.c	3.84% <ø> (ø)	⬆️
enc/windows_1250.c	21.27% <ø> (ø)	⬆️
enc/iso_8859_8.c	0% <ø> (ø)	⬆️
enc/unicode.c	71.47% <ø> (ø)	⬆️
enc/windows_1251.c	23.8% <ø> (ø)	⬆️
enc/euc_jp.c	91.8% <ø> (ø)	⬆️
... and 25 more

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