Fix driver build by including an appropriate Requires.private line on pixman.

We'd previously been substituting PIXMAN_CFLAGS, but we've got a better tool
now, plus I deleted the PIXMAN_CFLAGS substitution without noticing.

xorg-server.pc.in

@@ -8,5 +8,6 @@ sdkdir=@sdkdir@
 Name: xorg-server
 Description: Modular X.Org X Server
 Version: @PACKAGE_VERSION@
-Cflags: -I${sdkdir} @PIXMAN_CFLAGS@
+Requires.private: pixman-1
+Cflags: -I${sdkdir}
 Libs: -L${libdir}