Fix CMake Unity builds

transcoder/basisu_astc_helpers.h has a pragma once, but it is also an
implementation header (when ASTC_HELPERS_IMPLEMENTATION is defined).

pragma once is not necessary here as the header has a separate ifdef
guard, and it blocks attempts to compile Basis as a CMake Unity build
(CMAKE_UNITY_BUILD=ON) or as a single-translation unit non-CMake build
if the helpers header has been included earlier in the TU, as the second
inclusion with implementation defined doesn't actually include the
header.

transcoder/basisu_astc_helpers.h

@@ -1,6 +1,5 @@
 // basisu_astc_helpers.h
 // Be sure to define ASTC_HELPERS_IMPLEMENTATION somewhere to get the implementation, otherwise you only get the header.
-#pragma once
 #ifndef BASISU_ASTC_HELPERS_HEADER
 #define BASISU_ASTC_HELPERS_HEADER