scripts/clean-includes: Enhance to handle header files

scripts/clean-includes: Enhance to handle header files

Enhance clean-includes to handle header files as well as .c source
files. For headers we merely remove all the redundant #include
lines, including any includes of qemu/osdep.h itself.

There is a simple mollyguard on the include file processing to
skip a few key headers like osdep.h itself, to avoid producing
bad patches if the script is run on every file in include/.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>