build: Make distclean remove possible leftovers.

The build system, as it is, removes only files it knows about in make
clean, which can break build when you pull from git and some files were
moved or deleted. Then the object files and dep files for these files
will stay possibly breaking the build.

This commit makes the distclean target remove all files that looks like
they were generated, i.e. *.o *.dep *.gen.h and *.gen.c.

Reported-by: Milan Vancura <milan@ucw.cz>
Signed-off-by: Cyril Hrubis <metan@ucw.cz>