Fixed the sign compare causing build failure (AcademySoftwareFoundation#4240)

This is a trivial fix for build with warnings treated as errors.

Signed-off-by: Peter Kovář <peter.kovar@reflexion.tv>
Signed-off-by: Scott Wilson <scott@propersquid.com>

Author: 1div0

src/jpegxl.imageio/jxloutput.cpp

@@ -187,7 +187,7 @@ JxlOutput::open(const std::string& name, const ImageSpec& newspec,
     JxlEncoderSetBasicInfo(m_encoder.get(), &m_basic_info);
 
     if (m_basic_info.num_extra_channels > 0) {
-        for (int i = 0; i < m_basic_info.num_extra_channels; i++) {
+        for (uint32_t i = 0; i < m_basic_info.num_extra_channels; i++) {
             JxlExtraChannelType type = JXL_CHANNEL_ALPHA;
             JxlExtraChannelInfo extra_channel_info;