xcb-proto's pkg-config file exports static file paths for where to find
its Python files and the XML definitions.

When used to discover cflags, library paths, etc, pkg-config will
prepend ${PKG_CONFIG_SYSROOT_DIR}, if any, to those paths. This makes
the use of a sysroot prefix transparent to users. However, since it
doesn't know about paths in custom variables, it cannot automatically
prefix this.

Adding ${pc_sysrootdir} to these absolute paths makes the behaviour
match, and XCB builds work out of the box in a sysroot.
Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>