Fix gtk-doc build, Debian bug #1003075

Add a description of the new feature/bug fix. Reference any relevant bugs.

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated / NEWS entry present (for non-trivial changes)
• CI timeout is 2h or higher (see Settings/CICD/General pipelines/Timeout)

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code

Hello,

gtk-doc has been failing silently since for quite some time, any release tarball since 3.6.7 included empty reference deocumentation:

gnutls-3.6.6/doc/reference/html/:
api-index-full.html   gnutls-pkcs11.html    left.png
gnutls-abstract.html  gnutls-pkcs12.html    right-insensitive.png
gnutls-crypto.html    gnutls-tpm.html       right.png
gnutls.devhelp2       gnutls-x509.html      style.css
gnutls-dtls.html      home.png              up-insensitive.png
gnutls-gnutls.html    index.html            up.png
gnutls-ocsp.html      intro.html
gnutls-openpgp.html   left-insensitive.png

gnutls-3.6.7/doc/reference/html/:
api-index-full.html  intro.html             right.png
gnutls.devhelp2      left-insensitive.png   style.css
home.png             left.png               up-insensitive.png
index.html           right-insensitive.png  up.png

This was found, diagnosed and fixed by Dennis Filder, I have just tested it, split the patch and generated the merge-request.