package/stress-ng: Fix build issue with libbsd wchar check

stress-ng uses test/test-bsd-wchar.c program to check if there is
support for libbsd's wchar, but the test fails because wchar.h also
requires the FILE* definition from stdio.h which is not included
automatically, so fix it by include stdio.h explicitly.

Fixes:
	http://autobuild.buildroot.net/results/6def1bba8e7ec05682e74c4edc3a35c8c118d28b



Signed-off-by: Vadim Kochan <vadim4j@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>