ppd42ns: glibc < 2.17 need to link to lrt to use clock_gettime()

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
Signed-off-by: Sarah Knepper <sarah.knepper@intel.com>

src/ppd42ns/CMakeLists.txt

@@ -2,4 +2,4 @@ set (libname "ppd42ns")
 set (libdescription "upm ppd42ns dust sensor module")
 set (module_src ${libname}.cxx)
 set (module_h ${libname}.h)
-upm_module_init()
+upm_module_init("-lrt")