gettext: fix tools build failure for uClibc toolchain with C++

Fixes:
http://autobuild.buildroot.net/results/e5777d754653edef9d8c8558f04c2836734f2c25/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>