Documentation/sphinx: use a more sensible string split in kernel-doc extension

Using the default str.split doesn't return empty strings like the
current version does.
Signed-off-by: NJani Nikula <jani.nikula@intel.com>

Documentation/sphinx/kernel-doc.py

@@ -68,7 +68,7 @@ class KernelDocDirective(Directive):
         elif 'doc' in self.options:
             cmd += ['-function', str(self.options.get('doc'))]
         elif 'functions' in self.options:
-            for f in str(self.options.get('functions')).split(' '):
+            for f in str(self.options.get('functions')).split():
                 cmd += ['-function', f]
 
         cmd += [filename]