sdl: Fix missing autoconf

Fixing an issue seen on the autobuilders with sdl failing to run the
autogen.sh script. Several of the autobuild issues are listed below.

http://autobuild.buildroot.net/results/7326631048bc9e15ffadc6f541f2235592b5259b/
http://autobuild.buildroot.net/results/0adf5ea64075323eba1183f93c16d2954673f3e1/



Signed-off-by: Clayton Shotwell <clshotwe@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>