atftp: fix build with gcc5

atftp: fix build with gcc5

gcc5 defaults to -std=gnu11 which changes the inline semantics. This leads to
multiple definition error with 'extern inline'. See
https://gcc.gnu.org/gcc-5/porting_to.html for the details.

Fixes:
http://autobuild.buildroot.net/results/762/762bce2bbc1f994cc796f0442bfa73b7ad64e272/
http://autobuild.buildroot.net/results/28b/28bf6018dcf91ddacfb76f54a92313da8bf182e1/
http://autobuild.buildroot.net/results/fde/fdee23ecd0c5c66fda1ed38efe43048ce934b733/

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>