fix: include order when building with -DCXXOPTS_USE_UNICODE

This change fixes a bizarre build failure that happens when trying to
build docling-parse with system cxxopts that define `CXXOPTS_USE_UNICODE`.

The error causes implicit conversion from boolean types to fail, causing
errors like this:

```
In file included from /build/docling_parse-5.7.0/src/parse.h:44,
                 from /build/docling_parse-5.7.0/app/parse_fonts.cpp:17:
/build/docling_parse-5.7.0/src/parse/config.h: In member function ‘nlohmann::json_abi_v3_12_0::json pdflib::decode_config::to_json() const’:
/build/docling_parse-5.7.0/src/parse/config.h:60:58: error: no matching function for call to ‘nlohmann::json_abi_v3_12_0::basic_json<>::basic_json(const bool&)’
   60 |     j["do_sanitization"] = nlohmann::json(do_sanitization);
      |                                                          ^
```

Signed-off-by: Attila Oláh <attila@dorn.haus>

Author: attilaolah

src/parse.h

@@ -22,14 +22,16 @@
 #endif
 
 // specific libraries
+// NOTE: json.hpp must be included *before* cxxopts.hpp.
+// See https://github.com/docling-project/docling-parse/pull/248 for details.
+#include <nlohmann/json.hpp>
 #include <cxxopts.hpp>
 
 #define LOGURU_WITH_STREAMS 1
 #include <loguru.hpp>
 
 //#include <utf8/utf8.h>
 #include <utf8.h>
-#include <nlohmann/json.hpp>
 
 #define POINTERHOLDER_TRANSITION 0 // eliminate warnings from QPDF
 #include <qpdf/QPDF.hh>