Hide OpenSSL headers in private module

[kishikawakatsumi]

Fixes #3
Fixes #17

No longer needed to add header search paths in a user project.

[codecov]

Codecov Report

Merging #20 into master will decrease coverage by 0.13%.
The diff coverage is 72.54%.

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
- Coverage   50.16%   50.03%   -0.14%     
==========================================
  Files          23       24       +1     
  Lines        3026     2984      -42     
  Branches       41       45       +4     
==========================================
- Hits         1518     1493      -25     
+ Misses       1508     1491      -17

Impacted Files	Coverage Δ
BitcoinKit/HDPublicKey.swift	34.54% <0%> (+15.73%)	⬆️
BitcoinKit/Mnemonic.swift	63.46% <100%> (-3.21%)	⬇️
BitcoinKit/Serialization.swift	46.96% <100%> (ø)	⬆️
BitcoinKit/Crypto.swift	100% <100%> (ø)	⬆️
BitcoinKit/PrivateKey.swift	83.95% <100%> (-1.77%)	⬇️
BitcoinKit/PublicKey.swift	85.71% <100%> (-7.62%)	⬇️
BitcoinKit/BitcoinKitInternal.m	71.42% <71.42%> (ø)
BitcoinKit/HDPrivateKey.swift	92.59% <90.9%> (-1.1%)	⬇️

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