-lz in protobuf.pc belongs in Libs.private, not Libs

protobuf.pc.in

@@ -6,7 +6,8 @@ includedir=@includedir@
 Name: Protocol Buffers
 Description: Google's Data Interchange Format
 Version: @VERSION@
-Libs: -L${libdir} -lprotobuf @LIBS@ @PTHREAD_CFLAGS@ @PTHREAD_LIBS@
+Libs: -L${libdir} -lprotobuf @PTHREAD_CFLAGS@ @PTHREAD_LIBS@
+Libs.private: @LIBS@
 Cflags: -I${includedir} @PTHREAD_CFLAGS@
 # Commented out because it crashes pkg-config *sigh*:
 #   http://bugs.freedesktop.org/show_bug.cgi?id=13265