src: lcrypto: fix compiler warning

../deps/luacrypto/src/lcrypto.c:1064:11: warning: comparison of integers of different signs: 'int'
      and 'size_t' (aka 'unsigned long') [-Wsign-compare]
  if (ret != strlen(key)) {
      ~~~ ^  ~~~~~~~~~~~
1 warning generated.
Merge branch 'fix-compiler-warning' of https://github.com/racker/luacrypto into racker-fix-compiler-warning

Conflicts:
	src/lcrypto.c