build: add required libraries to libuv.pc.in

Example file after patch:

~~~~
$ cat libuv.pc
prefix=/usr/local
exec_prefix=/usr/local
libdir=${exec_prefix}/lib
includedir=${prefix}/include

Name: libuv
Version: 0.11.22
Description: multi-platform support library with a focus on asynchronous I/O.

Libs: -L${libdir} -luv -lrt -lpthread -lnsl -ldl 
Cflags: -I${includedir}
(pkg_config)saghul@alpha:~/src/libuv
~~~~

Note the "Libs" line.