resource-encapsulation: fix build without curl

resource-encapsulation: fix build without curl

The hue plugin needs curl and it should only be build when libcurl is available.
Without this patch, the build fails without curl installed, this patch fixes the problem and makes the build worm without curl and without the hue stuff.

Without this patch I am getting this error message:
scons: ***
File "/service/third_party_libs.scons", line 50, in ?

Change-Id: I5b4da555ff84b9b605cc6c119990d60ff670bd0d
Signed-off-by: Hauke Mehrtens <hauke.mehrtens@lantiq.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/2199
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Minji Park <minjii.park@samsung.com>
Reviewed-by: Markus Jung <markus.jung@samsung.com>
Reviewed-by: Uze Choi <uzchoi@samsung.com>