pkgconf: Add yuv library in Libs.private for static linking (#1906)

libyuv was not added in Requires.private because it does not provide pkgconfig file officially

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 9aaa78c..60a9c18 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -43,6 +43,7 @@
 )
 option(AVIF_ENABLE_EXPERIMENTAL_AVIR "Enable experimental reduced header" OFF)
 
+set(AVIF_PKG_CONFIG_EXTRA_LIBS_PRIVATE "")
 set(AVIF_PKG_CONFIG_EXTRA_REQUIRES_PRIVATE "")
 
 function(set_local_or_system_option VAR DEFAULT TEXT)
@@ -222,6 +223,7 @@
 if(AVIF_LIBYUV_ENABLED)
     target_compile_definitions(avif_obj PRIVATE -DAVIF_LIBYUV_ENABLED=1)
     avif_target_link_library(yuv::yuv)
+    set(AVIF_PKG_CONFIG_EXTRA_LIBS_PRIVATE "${AVIF_PKG_CONFIG_EXTRA_LIBS_PRIVATE} -lyuv")
 endif(AVIF_LIBYUV_ENABLED)
 
 set_local_or_system_option("LIBSHARPYUV" "OFF" "Use libsharpyuv.")

libavif.pc.cmake

diff --git a/libavif.pc.cmake b/libavif.pc.cmake
index bab05f8..1341c2b 100644
--- a/libavif.pc.cmake
+++ b/libavif.pc.cmake
@@ -7,6 +7,7 @@
 Description: Library for encoding and decoding .avif files
 Version: @PROJECT_VERSION@
 Libs: -L${libdir} -lavif
+Libs.private:@AVIF_PKG_CONFIG_EXTRA_LIBS_PRIVATE@
 Cflags: -I${includedir}@AVIF_PKG_CONFIG_EXTRA_CFLAGS@
 Cflags.private: -UAVIF_DLL
 Requires.private:@AVIF_PKG_CONFIG_EXTRA_REQUIRES_PRIVATE@