Fix cmake LIST cache variables - LIST is not a valid cache type. (#17971)

This doesn't affect the behaviour of UR_EXTERNAL_DEPENDENCIES - since it
gets used as a list, if you try to set it to a string that cmake can't
convert to a list you get a fatal error (e.g. doing
`set(UR_EXTERNAL_DEPENDENCIES "my-dep my-other-dep")` rather than
`set(UR_EXTERNAL_DEPENDENCIES "my-dep;my-other-dep")`).

The list of valid cache variable types is here
https://cmake.org/cmake/help/latest/prop_cache/TYPE.html

Author: aarongreig

CMakeLists.txt

@@ -56,7 +56,7 @@ option(UR_BUILD_XPTI_LIBS "Build the XPTI libraries when tracing is enabled" ON)
 option(UR_STATIC_LOADER "Build loader as a static library" OFF)
 option(UR_FORCE_LIBSTDCXX "Force use of libstdc++ in a build using libc++ on Linux" OFF)
 option(UR_ENABLE_LATENCY_HISTOGRAM "Enable latncy histogram" OFF)
-set(UR_EXTERNAL_DEPENDENCIES "" CACHE LIST
+set(UR_EXTERNAL_DEPENDENCIES "" CACHE STRING
     "List of external CMake targets for executables/libraries to depend on")
 set(UR_DPCXX "" CACHE FILEPATH "Path of the DPC++ compiler executable")
 set(UR_DPCXX_BUILD_FLAGS "" CACHE STRING "Build flags to pass to DPC++ when compiling device programs")