Fix cstest build with Ninja

[thestr4ng3r]

Your checklist for this pull request

• I've documented or updated the documentation of every API function and struct this PR changes.
• I've added tests that prove my fix is effective or that my feature works (if possible)

Detailed description

When building with Ninja, it is necessary to explicitly declare that the libcmocka.a and libcyaml.a files are built by their respective ExternalProject targets, otherwise it will refuse to build.

See also the documentation of BUILD_BYPRODUCTS in https://cmake.org/cmake/help/v3.31/module/ExternalProject.html#id13

Test plan

The following should not error:

florian-macbook:capstone florian$ cmake -Bbuild -GNinja -DCAPSTONE_BUILD_CSTEST=ON
-- The C compiler identification is AppleClang 16.0.0.16000026
-- The CXX compiler identification is AppleClang 16.0.0.16000026
(...)
-- Configuring done (0.6s)
-- Generating done (0.0s)
-- Build files have been written to: /Users/florian/dev/capstone/build
florian-macbook:capstone florian$ ninja -Cbuild
ninja: Entering directory `build'
ninja: error: 'suite/cstest/extern/src/cmocka_ext-build/src/libcmocka.a', needed by 'suite/cstest/cstest', missing and no known rule to make it

[kabeor]

Thank you! Merged.