RFR: Add missing inputs to tasks in gradle build

[Jorn Vernee]

I recently re-did some of the task dependencies of the gradle build: https://github.com/openjdk/jextract/pull/200

>From my initial testing for that PR, and reading online, I was under the impression that tasks would re-run when the outputs of a dependency changes. After additional use & testing, this turns out to be not the case and we need to explicitly set task inputs any way. This PR does that. 

I also changed the `copyLibClang` task from `Copy` to `Sync`. If a file is removed from the source directory, `Copy` will not remove it from the output, but `Sync` will. This helps ensure that we don't package any stale files into the jextract jmod (which might create include conflicts). This is also something I noticed during more thorough testing.

Finally, I noticed that on Windows we were copying `clang.exe` to the jextract jmod archive, since we copy from the `bin` dir on Windows, which contains both the `libclang.dll` and the `clang.exe` file. I've added an exclude for `clang.exe`.

-------------

Commit messages:
 - exclude clang.exe
 - Fix build again

Changes: https://git.openjdk.org/jextract/pull/204/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=204&range=00
  Stats: 6 lines in 1 file changed: 5 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jextract/pull/204.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/204/head:pull/204

PR: https://git.openjdk.org/jextract/pull/204