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>

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