Add missing files to release artifacts

[falbrechtskirchinger]

Add missing files to include.zip:

• amalgamated json_fwd.hpp
• Bazel build files

Fixes #3727.

[falbrechtskirchinger]

$ make include.zip
(...)
  adding: single_include/nlohmann/json_fwd.hpp (deflated 74%)
  adding: BUILD.bazel (deflated 81%)
  adding: WORKSPACE.bazel (stored 0%)

[falbrechtskirchinger]

(Let's wait on #3709.)

[coveralls]

Coverage remained the same at 100.0% when pulling 2b3a1d4b74fca6b847ac0eb143b1ceb10374c670 on falbrechtskirchinger:add-missing-release-files into f4658de2701f2290baf076bbbd8540260015518c on nlohmann:develop.

[nlohmann]

Thanks!