st: Fix generated GLSL dependency
StScrollViewFade depends on st-scroll-view-fade-generated.c, but that dependency isn't expressed to the build system; we just hope that the custom target runs before compiling the effect.
Instead, add the generated source to the st target so the dependency is expressed properly.
(The change from .c to .h is to prevent the file from being both included and compiled, resulting in a duplicated symbol)