package/sdl2_ttf: x-includes and x-libraries must be set for cross-compiling

package/sdl2_ttf: x-includes and x-libraries must be set for cross-compiling

If those flags are not explicitly passed, the sdl2_ttf configure script
will include -I/usr/include and -L/usr/lib in the compile flags, which
are obviously unsafe for cross-compilation.

"checking for X... libraries /usr/lib, headers /usr/include"

Fixes:

  http://autobuild.buildroot.net/results/f968fb31e9882cc856296a7c729d93e345e9e861

Signed-off-by: Romain Naour <romain.naour@gmail.com>
[Thomas:
 - don't pass an argument to --with-x
 - use --without-x instead of --with-x=no]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>