android: c_compat: add __cmsg_nxthd() for Android KitKat 4.4

android: c_compat: add __cmsg_nxthd() for Android KitKat 4.4

The function __cmsg_nxthd() is missing in Android 4.4, but the Android NDK
header files in the version we are using are referencing it and we use it in
our code, this functions was added in version 5.0. To make IoTivity
dynamically loadable on Android KitKat 4.4 add this functions
as a weak symbol, so it will be used if the c lib does not provide it, like
on Android < 5.0 This code was taken from these two resources:
https://raw.githubusercontent.com/android/platform_bionic/master/libc/bionic/__cmsg_nxthdr.cpp
https://github.com/android/platform_bionic/commit/ff64831b0965c16c95c9f81a148f30a6ef3a6c64

Change-Id: I02ef763cf3f82c73b1e0169230dd8c7e9d6d379b
Signed-off-by: Hauke Mehrtens <hauke.mehrtens@lantiq.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/3895
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Jon A. Cruz <jonc@osg.samsung.com>