fix compilation error
fix issue #213
The problem seems to occur when sdl-gpu is installed, then CMakeList
was calling include_directories cmake function with SDL path before adding
project include/, So /usr/include/SDL had an higher priority than
sdl-gpu includes, and so it was using old versions,
which didn't have default_textured_vertex_shader_id.
Signed-off-by: matthias <uso.cosmo.ray@gmail.com>