corrected header search logic

Corrected header search logic:

these files search correctly for the custom platform header:

lib\loader.c(48): #include "KHR/khrplatform.h"
lib\swap.c(35): #include "KHR/khrplatform.h"

but ktx.h didn't and this way caught the wrong one from /usr/...

include\ktx.h(134): #include <KHR/khrplatform.h>

Corrected that.