kernel-doc: add support for specifying extra files for EXPORT_SYMBOLs

kernel-doc: add support for specifying extra files for EXPORT_SYMBOLs

If the kernel-doc comments for functions are not in the same file as the
EXPORT_SYMBOL statements, the -export and -internal output selections do
not work as expected. This is typically the case when the kernel-doc
comments are in header files next to the function declarations and the
EXPORT_SYMBOL statements are next to the function definitions in the
source files.

Let the user specify additional source files in which to look for the
EXPORT_SYMBOLs using the new -export-file FILE option, which may be
given multiple times.

The pathological example for this is include/net/mac80211.h, which has
all the kernel-doc documentation for the exported functions defined in a
plethora of source files net/mac80211/*.c.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>