Fix missing prototype for newlineScanByteCountForTesting (#1720)

Closes #1705. Adds explicit prototype declaration for
newlineScanByteCountForTesting() to prevent compiler errors when
building with -Wmissing-prototypes.

Signed-off-by: Tyagiquamar <mohdquamartyagi@gmail.com>
Co-authored-by: Jordan Bayles <bayles.jordan@gmail.com>

src/lib_json/json_reader.cpp

diff --git a/src/lib_json/json_reader.cpp b/src/lib_json/json_reader.cpp
index 2f901f5..39af2c4 100644
--- a/src/lib_json/json_reader.cpp
+++ b/src/lib_json/json_reader.cpp
@@ -988,6 +988,7 @@
 // CharReaderTest/parseCommentsAfterValueScansLinearly). thread_local so it
 // never races during concurrent parsing; the increment is negligible and only
 // runs while parsing comments. Not part of the supported public API.
+JSON_API size_t& newlineScanByteCountForTesting();
 JSON_API size_t& newlineScanByteCountForTesting() {
   static thread_local size_t count = 0;
   return count;