Fix the license header of hkdf

[RonEld]

Change the license header of hkdf.h to a format the that script
apache_to_gpl.pl knows how to parse.

[jainvikas8]

Please could you let me know how to reproduce this issue? (invoke the script - apache_to_gpl.pl)
Also there seems to be failure in TLS testing

test_no_use_psa_crypto_full_cmake_asan: test: ssl-opt.sh (full minus >MBEDTLS_USE_PSA_CRYPTO): tests/ssl-opt.sh -> 1
Is this an known issue?

The copyright year information, Is that allowed?

[RonEld]

@jainvikas8
The script that changes the header to GPL is in our web site repository.
It is generate_license.pl , however internally, apache_to_gpl.pl is called, which does the parsing of the header.

The copyright year information, Is that allowed?

Well, AFAIK, in Apache, every time a file is modified, its copyright year should be modified as well. Although I don't consider this change significant enough to merit an update in copyright year, this file has been changed during 2019 in other commits, so I modified the year as well. In fact, I am not sure the starting year should be 2016, as this file was introduced on 2018.

[RonEld]

steps to reproduce:

1. cp hkdf.h and another header file to a temporary location
2. Run the apache_to_gpl.pl script:

./apache_to_gpl.pl . <hkdf.h file path> 

The license header will not change
3. . Run the same script on the other header file, and you will see the license header has changed

[jainvikas8]

steps to reproduce:

1. cp `hkdf.h` and another header (fixed) file to a temporary location

2. Run the `apache_to_gpl.pl` script:

./apache_to_gpl.pl . <hkdf.h file path> 

The license header will not change
3. . Run the same script on the other header (fixed) file, and you will see the license header has changed

Additional note : This issue is seen on Mbed TLS 2.16.2 Apache.

[RonEld]

test_no_use_psa_crypto_full_cmake_asan: test: ssl-opt.sh (full minus >MBEDTLS_USE_PSA_CRYPTO): tests/ssl-opt.sh -> 1
Is this an known issue?

Yes, this is a known flaky test issue with Gnu TLS DTLS client

[RonEld]

I updated the copyright year to reflect the correct years that this file has been modified.
@jainvikas8 @Patater please review

[jainvikas8]

Looks fine to me, able to reproduce the issue. @Patater please comment on the licensing year. Thanks.

[RonEld]

@jainvikas8 As answered by legal, the copyright year should reflect every year the file was modified. This means that in this hkdf.h file, it should be 2018-2019 as modified in this PR