🐛 add missing header to input_adapters.hpp

[ddkilzer]

While the WebKit project was in the process of adopting Swift (clang) modules, a build error occurred because json.hpp uses std::streambuf, but it doesn't include the <streambuf> header. (A dependent project included json.hpp, then WebKit included this project's header, which is how it was found.)

The fix is to add the missing header to input_adapters.hpp.

• The changes are described in detail, both the what and why.
• If applicable, an existing issue is referenced.
• The Code coverage remained at 100%. A test case for every new line of code.
• If applicable, the documentation is updated.
• The source code is amalgamated by running make amalgamate.

[nlohmann]

Looks good to me.

[coveralls]

coverage: 99.19%. remained the same
when pulling 4d7ba33 on ddkilzer:include-streambuf-in-input_adapters.hpp
into b451735 on nlohmann:develop.

[nlohmann]

Thanks!