libzmq: Fix pkg-config files for static linking

Libzmq uses C++ standard library features, so users of it should link
against that as well when statically linking.

Add it to Libs.private so users using pkg-config automatically gets the
correct linker flags.
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

src/libzmq.pc.cmake.in

@@ -7,4 +7,5 @@ Name: libzmq
 Description: 0MQ c++ library
 Version: @ZMQ_VERSION_MAJOR@.@ZMQ_VERSION_MINOR@.@ZMQ_VERSION_PATCH@
 Libs: -L${libdir} -lzmq
+Libs.private: -lstdc++
 Cflags: -I${includedir}

src/libzmq.pc.in

@@ -7,4 +7,5 @@ Name: libzmq
 Description: 0MQ c++ library
 Version: @VERSION@
 Libs: -L${libdir} -lzmq
+Libs.private: -lstdc++
 Cflags: -I${includedir}