build: add url field to libuv.pc

PR-URL: https://github.com/libuv/libuv/pull/1733
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

libuv.pc.in

@@ -6,6 +6,7 @@ includedir=@includedir@
 Name: @PACKAGE_NAME@
 Version: @PACKAGE_VERSION@
 Description: multi-platform support library with a focus on asynchronous I/O.
+URL: http://libuv.org/
 
 Libs: -L${libdir} -luv @LIBS@
 Cflags: -I${includedir}