Add missing prototype for newlineScanByteCountForTesting

[afonsojanu]

Fixes #1705.

newlineScanByteCountForTesting() in json_reader.cpp is defined with no prior declaration in the same translation unit. Building with stricter warning flags (clang's -Wmissing-prototypes, as reported from the OpenXR SDK build) flags it as a possible externally-callable function with no visible prototype, even though it's test-only instrumentation (main.cpp forward-declares it itself before using it, since it's not part of any installed header).

Adds a plain prototype directly above the definition, matching what you suggested in the issue. This silences the warning without changing the public API surface at all: the function still isn't declared anywhere in include/.

Verified with a standalone clang++ -Wmissing-prototypes -Werror compile of json_reader.cpp: fails with the exact error from the issue on unmodified master, passes with this change. Also reran the full meson/ninja test suite (unittest_jsoncpp_test, unittest_jsontestrunner, jsonchecker_jsontestrunner), all green, and ran clang-format per CONTRIBUTING.md with no further diff.

[greptile-apps]

Greptile Summary

Adds a forward declaration for the test-only newlineScanByteCountForTesting() function so strict missing-prototype builds succeed without expanding the installed public API.

• Declares the function immediately before its existing definition.
• Preserves the function’s signature, visibility annotation, and behavior.

Confidence Score: 5/5

The PR appears safe to merge with no actionable issues identified.

The added declaration exactly matches the adjacent existing definition and only addresses strict compiler prototype diagnostics without altering behavior or the installed API.

Important Files Changed

Filename	Overview
src/lib_json/json_reader.cpp	Adds a matching prototype for existing test instrumentation; no runtime behavior or public-header surface changes.

_{Reviews (1): Last reviewed commit: "Add missing prototype for newlineScanByt..." | Re-trigger Greptile}