Add dependency on dexpreopt.config to soong_build doc generation.

This was caught while running the soong_build docs action under Bazel as
ninja executor. Since the config file is not an explicit input to the
action, it's not present in Bazel's execution root during action
execution time, leading to a missing file error. To fix it, add the file
as an input to the action, if it's configured in product variables.

New soong_build statement:

build out/soong/docs/soong_build.html: s.writedocs.soongDocs $
        out/.module_paths/Android.bp.list out/soong/soong.variables $
        out/soong/dexpreopt.config | out/soong/.bootstrap/bin/soong_build
    outDir = out/soong/docs

Test: m
Test: USE_BAZEL=1 m
Bug: 161958189
Fixes: 161958189
Change-Id: I2bf264e848bb29ac91f4196fbe9c4ecf3cea8a2c