Fix build on AIX failing because of access to string elements

The compilation failed with this error message:

.../Source/cmCryptoHash.cxx: In method `string cmCryptoHash::HashString (const string &)':
.../Source/cmCryptoHash.cxx:41: non-lvalue in unary `&'

This was introduced in 77f60392d93f41a8828d3db1d7b76d45f1535d07 (stringapi:
Accept strings when MD5 hashing data).
Merge topic 'aix-compile' into next

539b6c58 Fix build on AIX failing because of access to string elements