libcoap-1.pc.in: do not unnecessarily expose DTLS libs to users

The SSL lib used to implement DTLS is only used internally
by libcoap, so it is not necessary and sometimes harmful to
expose it via the pkg-config file. Only when doing static
linking is it necessary to link the SSL lib, this is done
via the "Libs.private: pkg-config directive.

libcoap-1.pc.in

@@ -7,5 +7,6 @@ Name: @PACKAGE_NAME@
 Description: C-Implementation of CoAP.
 Version: @PACKAGE_VERSION@
 URL: @PACKAGE_URL@
-Libs: -L${libdir} -lcoap-@LIBCOAP_API_VERSION@ @DTLS_LIBS@
-Cflags: -I${includedir} @DTLS_CFLAGS@
+Libs: -L${libdir} -lcoap-@LIBCOAP_API_VERSION@
+Libs.private: @DTLS_LIBS@
+Cflags: -I${includedir}