libtpmtss: Read RSA public key exponent instead of assuming its value

libtpmtss: Read RSA public key exponent instead of assuming its value

Up to now it was assumed that the RSA public key exponent is equal to 2^16+1.
Although this is probably true in most if not all cases, it is not correct
according to the TPM 2.0 specification.

This patch fixes that by reading the exponent from the structure returned
by TPM2_ReadPublic.

Closes strongswan/strongswan#121.