package/smstools3: fix for recent make

smstools3 defines variables in the middle of a recipe, which
make-3.81 accepts, but make-3.82 and later whine about.

Fixes:
    http://autobuild.buildroot.net/results/7fa/7fa839c7268256cb4f3b8728b3f50818cf3d3670/



Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>