cmake: use zephyr_syscall_include_directories

Instead of appending to SYSCALL_INCLUDE_DIRS.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>

Author: gmarull

CMakeLists.txt

@@ -7,8 +7,7 @@
 
 # This is needed so that custom driver classes using system calls are taken into
 # account
-list(APPEND SYSCALL_INCLUDE_DIRS ${CMAKE_CURRENT_SOURCE_DIR}/include)
-set(SYSCALL_INCLUDE_DIRS ${SYSCALL_INCLUDE_DIRS} PARENT_SCOPE)
+zephyr_syscall_include_directories(include)
 
 zephyr_include_directories(include)