This is my first pull request here - sorry if something does not fit to the rules

Description

Changed: Always add Qt5::Gui to build, needed because {{prefix}}Oauth.h is always part of the build
Added: find_package for OpenSSL, should be done prior target_link_libraries

Tests that have been done :

(ubuntu)
install deps (reference #11371 ):
sudo apt install -y --no-install-recommends qt5-default
run code generator:
java -jar openapi-generator-cli-5.4.0.jar generate -t cpp-qt-templates -i petstore.json -g cpp-qt-client -o petstore
run cmake:
cmake -S petstore -B petstore/build

(windows)
run code generator:
java -jar openapi-generator-cli-5.4.0.jar generate -t cpp-qt-templates -i petstore.json -g cpp-qt-client -o petstore
run cmake:
cmake -S petstore -B petstore/build

Issue

fixes #11371

technical commitee

@muttleyxd
@etherealjoy
@wing328

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
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  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (5.3.0), 6.0.x
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