cmake: extensions: introduce zephyr_syscall_include_directories

Add a new CMake helper to add new include directories to be parsed by
the syscall machinery. This helper complements the existing
zephyr_syscall_header, which works at a header-level.

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

CMakeLists.txt

@@ -694,6 +694,13 @@ if(CONFIG_ZTEST)
 
 endif()
 
+get_property(
+  syscalls_include_list
+  TARGET syscalls_interface
+  PROPERTY INTERFACE_INCLUDE_DIRECTORIES
+)
+list(APPEND SYSCALL_INCLUDE_DIRS ${syscalls_include_list})
+
 foreach(d ${SYSCALL_INCLUDE_DIRS})
   list(APPEND parse_syscalls_include_args
     --include ${d}