aws-c-event-stream: fix cmake imported target name

[SpaceIm]

Specify library name and version: aws-c-event-stream/all

• I've read the guidelines for contributing.
• I've followed the PEP8 style guides for Python code in the recipes.
• I've used the latest Conan client version.
• I've tried at least one configuration locally with the
  conan-center hook activated.

Moreover, since aws-c-event-stream depends on aws-c-common and aws-checksums, #2582 and #2583 would avoid using conan targets (CONAN_PKG::) in this recipe.

[conan-center-bot]

All green in build 1 (80ba837b30626fb62f9804d18086869f684a0862)! 😊

• aws-c-event-stream/0.1.5: Generated 82 packages. All logs here