package/mosquitto: fix static build with openssl

package/mosquitto: fix static build with openssl

mosquitto library can be built statically since commit
a95bdd964761ec69a23174f5213dd54b77d0971b however build fails with
openssl because mosquitto does not use pkg-config to retrieve static
dependencies so provide them through the STATIC_LIB_DEPS variable

Fixes:
 - http://autobuild.buildroot.org/results/2ee69a5d7834c9a4845ebde801be6aa8f97cc5fa

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>