Changes to make "make distcheck" pass

Changes to make "make distcheck" pass

There were two errors:

1) Changelog wasn't deleted by "make distclean"

2) libsigrok.h wasn't treated as a distributable file and hence couldn't be found. This is a result of two commits:

https://sigrok.org/gitweb/?p=libserialport.git;a=commit;h=f1c916ede191ec398cbe53d7f978078311c25785 marking libsigrok.h as non-distributable since it was build from libsigrok.h.in at the time and

https://sigrok.org/gitweb/?p=libserialport.git;a=commit;h=f6e32b2dfa322aa4bd4a279a46d588e0e73ea238 which removed libsigrok.h.in in favor of a static libsigrok.h

The second commit didn't revert the Makefile.am change that marked libsigrok.h as non-distributable, which this commit takes care of.