libjxl : do not export "-lm" private dependency under Windows.

### Is your feature request related to a problem? Please describe.

Hi,

The usage of the libjxl under Windows is broken at linking time due to wrong pkgconfig settings exported by the library: -lm

This is the KF6::kimageformat compilation trace on my computer:

```
Automatic MOC for target kimg_jxl
Building Custom Rule E:/b/dk_cmake/ext_kf6/ext_kimageformats-prefix/src/ext_kimageformats/src/imageformats/CMakeLists.txt
jxl.cpp
mocs_compilation_RelWithDebInfo.cpp
Generating Code...
LINK : warning LNK4044: unrecognized option '/Wl,-Bsymbolic-functions'; ignored [E:\b\dk_cmake\ext_kf6\ext_kimageformats-prefix\src\ext_kimageformats-build\src\imageformats\kimg_jxl.vcxproj] [E:\b\dk_cmake\ext_kf6\ext_kimageformats.vcxproj]
LINK : fatal error LNK1104: cannot open file 'm.lib' [E:\b\dk_cmake\ext_kf6\ext_kimageformats-prefix\src\ext_kimageformats-build\src\imageformats\kimg_jxl.vcxproj] [E:\b\dk_cmake\ext_kf6\ext_kimageformats.vcxproj]
```

NOTE: another WM with KF5 compilation give exactly the same problem.

"-lm" is only for Linux for the Math library, not for Windows.

### Proposed solution

I patched all libjxl pkgconfig files (debug and non debug targets) and now all link fine. For ex, the "libjxl.pc" installed by VCPKG for the non-debug is this one:

```
prefix=${pcfiledir}/../..
exec_prefix=${prefix}
libdir=${exec_prefix}/lib
includedir=${prefix}/../include

Name: libjxl
Description: Loads and saves JPEG XL files
Version: 0.11.1
Requires.private: libhwy libbrotlienc libbrotlidec libjxl_cms
Libs: "-L${libdir}" -ljxl
Libs.private: -lm
Cflags: "-I${includedir}"
Cflags.private: -DJXL_STATIC_DEFINE
```

I removed the line : Libs.private: -lm

And i do it also for all other libjxl non-debug pkgconfig files : "libjxl_cms.pc" and "libjxl_threads.pc"

Finally all debug pkgconfig files are also patched in the same way. In total 6 files patched.

My best

Gilles Caulier

### Describe alternatives you've considered

_No response_

### Additional context

2 VirtualBox VM running under Linux KUbuntu 24.04 with Windows 10 64 bits all updates installed and Visual Studio 2022 version 17.12.4