/usr/share/pkgconfig/xproto.pc is in x11proto-dev 2018.4-4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 | prefix=/usr
exec_prefix=${prefix}
libdir=${prefix}/lib
includedir=${prefix}/include
includex11dir=${prefix}/include/X11
Name: Xproto
Description: Xproto headers
Version: 7.0.32
Cflags: -I${includedir}
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