security/py-argon2-cffi-bindings: Add missing cmake build dependency

[olgeni]

Summary

The 26.1.0 update switched the build backend to scikit-build-core.
When no cmake binary is present, scikit-build-core adds cmake>=3.20
as a dynamic build requirement, so in a clean poudriere jail the
pep517 build stops with:

• Getting build dependencies for wheel... ERROR Unmet dependencies (checked against /usr/local/bin/python3.12): cmake>=3.20 wanted: >=3.20 found: not installed

devel/py-scikit-build-core only lists cmake in TEST_DEPENDS, so the
consumer has to pull it in. Add USES=cmake:indirect, as other
scikit-build-core consumers in the tree do.

Test Plan

poudriere testport, 14.4 amd64, nox11 set:

• unmodified main (26.1.0): fails in the build phase with the error above (also seen in a 15.1 amd64 bulk)
• with this change: build, stage-qa and check-plist succeed; scikit-build-core 1.0.3 configures with CMake 3.31.12 and compiles only _ffi.c (the ARGON2_CFFI_USE_SYSTEM path, linking the system libargon2)

portlint -AC and portclippy are clean.