compat/*.[ch]: remove extern from function declarations using spatch

compat/*.[ch]: remove extern from function declarations using spatch

In 554544276a (*.[ch]: remove extern from function declarations using
spatch, 2019-04-29), we removed externs from function declarations using
spatch but we intentionally excluded files under compat/ since some are
directly copied from an upstream and we should avoid churning them so
that manually merging future updates will be simpler.

In the last commit, we determined the files which taken from an upstream
so we can exclude them and run spatch on the remainder.

This was the Coccinelle patch used:

	@@
	type T;
	identifier f;
	@@
	- extern
	  T f(...);

and it was run with:

	$ git ls-files compat/\*\*.{c,h} |
		xargs spatch --sp-file contrib/coccinelle/noextern.cocci --in-place
	$ git checkout -- \
		compat/regex/ \
		compat/inet_ntop.c \
		compat/inet_pton.c \
		compat/nedmalloc/ \
		compat/obstack.{c,h} \
		compat/poll/

Coccinelle has some trouble dealing with `__attribute__` and varargs so
we ran the following to ensure that no remaining changes were left
behind:

	$ git ls-files compat/\*\*.{c,h} |
		xargs sed -i'' -e 's/^\(\s*\)extern \([^(]*([^*]\)/\1\2/'
	$ git checkout -- \
		compat/regex/ \
		compat/inet_ntop.c \
		compat/inet_pton.c \
		compat/nedmalloc/ \
		compat/obstack.{c,h} \
		compat/poll/

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>