build: avoid the use of strsignal() in split

build: avoid the use of strsignal() in split

... which is not available on some platforms,
and the replacement currently requires linking
with threading libraries.
* src/split.c (closeout): Remove the call to strsignal()
which is largely redundant anyway as sig2str()
is already used to map number to name in the error.
Reported by Bruno Haible on AIX 6.1 and 7.1