Added an option to include additional directories in embed_devicecode

[Waqar-ukaea]

Very minor change here but I wanted to make including headers from another library a little more convenient when compiling the slang code.

A new INCLUDE_DIRS argument has been added to the embed_devicecode() cmake function, allowing users to specify additional include directories that will be passed to slangc via -I flags during shader compilation. It just makes it slightly more convenient for me to include additional headers in my project. I can use project relative includes instead of filesystem relative paths:

#include "../geometry/dp_math.h" ---> OLD
#include "xdg/geometry/dp_math.h" ---> NEW

Example usage of the INCLUDE_DIRS argument:

    embed_devicecode(
    OUTPUT_TARGET
    ${device_code}
    HEADERS
    ${CMAKE_CURRENT_SOURCE_DIR}/include/xdg/gprt/shared_structs.h
    SOURCES
    ${CMAKE_CURRENT_SOURCE_DIR}/src/gprt/${device_code}.slang
    INCLUDE_DIRS
    ${CMAKE_CURRENT_SOURCE_DIR}/include
  )