Use sysroot prefix for pkgdatadir variable

The pc_sysroot is automatically added to cflags and libs but not
to 'pkg-config --variable'

Reviewed-by: Daniel Stone <daniels@collabora.com>
diff --git a/wayland-protocols.pc.in b/wayland-protocols.pc.in
index a26744c..379be06 100644
--- a/wayland-protocols.pc.in
+++ b/wayland-protocols.pc.in
@@ -1,6 +1,6 @@
 prefix=@prefix@
 datarootdir=@datarootdir@
-pkgdatadir=@datadir@/@PACKAGE@
+pkgdatadir=${pc_sysrootdir}@datadir@/@PACKAGE@
 
 Name: Wayland Protocols
 Description: Wayland protocol files