Generate shlibs control file for debian package

Without the control file, dpkg-shlibdeps won't recognize this package.
Depending packages created by library user would end up without
dependency on libpaho-mqtt.
Signed-off-by: NStanislav Ravas <ravas@tind.sk>

CMakeLists.txt

@@ -62,6 +62,8 @@ ENDIF()
 
 IF(PAHO_BUILD_DEB_PACKAGE)
     set(CMAKE_INSTALL_DOCDIR share/doc/libpaho-mqtt)
+    set(CPACK_DEBIAN_PACKAGE_GENERATE_SHLIBS ON)
+    set(CPACK_DEBIAN_PACKAGE_GENERATE_SHLIBS_POLICY ">=")
 ENDIF()
 
 ADD_SUBDIRECTORY(src)