pkgconf: add UAVIF_DLL for static linking with pkgconf (#1904)

allows for the usage of `pkgconf --static --cflags libavif' to produce
the proper CFLAGS for static linking. Relies on pkgconf rather than pkg-config.

libavif.pc.cmake

diff --git a/libavif.pc.cmake b/libavif.pc.cmake
index c091443..ac58c24 100644
--- a/libavif.pc.cmake
+++ b/libavif.pc.cmake
@@ -8,3 +8,4 @@
 Version: @PROJECT_VERSION@
 Libs: -L${libdir} -lavif
 Cflags: -I${includedir}@AVIF_PKG_CONFIG_EXTRA_CFLAGS@
+Cflags.private: -UAVIF_DLL