doc: Use correct path for kerneldoc2doxygen.pl

Refer to kerneldoc2doxygen.pl as doc/kerneldoc2doxygen.pl since doxygen
is run in the top-level source directory.

Signed-off-by: Pavel Roskin <proski@gnu.org>

doc/doxygen.conf

@@ -674,7 +674,7 @@ IMAGE_PATH             = doc
 # If FILTER_PATTERNS is specified, this tag will be
 # ignored.
 
-INPUT_FILTER           = kerneldoc2doxygen.pl
+INPUT_FILTER           = doc/kerneldoc2doxygen.pl
 
 # The FILTER_PATTERNS tag can be used to specify filters on a per file pattern
 # basis.