fix inccorect export of aws_symmetric_cipher_get_state

[sbiscigl]

Description of changes:

fixes a silly mistake i made in pull/184 where i made the declaration outside of the scope of AWS_EXTERN_C_END which causes the following linker error when used.

Undefined symbols for architecture arm64:
  ....
   NOTE: found '_aws_symmetric_cipher_get_state' in libaws-c-cal.1.0.0.dylib, declaration possibly missing 'extern "C"'
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

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