cmake: add zephyr_syscall_header stub

This adds a new cmake function zephyr_syscall_header() which
will be used in the future to limit the scope of syscalls
in the built binary. Currently this is just an empty stub
so that add zephyr_syscall_header() to kernel, subsys, etc.
without breaking the build.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

cmake/modules/extensions.cmake

@@ -1486,6 +1486,19 @@ function(zephyr_build_string outvar)
   set(${outvar} ${${outvar}} PARENT_SCOPE)
 endfunction()
 
+# Function to add header file(s) to the list to be passed to syscall generator.
+function(zephyr_syscall_header)
+  # Empty function for now. Will implement later.
+endfunction()
+
+# Function to add header file(s) to the list to be passed to syscall generator
+# if condition is true.
+function(zephyr_syscall_header_ifdef feature_toggle)
+  if(${${feature_toggle}})
+    # Empty function for now. Will implement later.
+  endif()
+endfunction()
+
 ########################################################
 # 2. Kconfig-aware extensions
 ########################################################