Fix build failure (conflicting decl of strnlen) and clean up

Fix build failure (conflicting decl of strnlen) and clean up

* parted/strlist.h: Include <config.h> first.
[!ENABLE_NLS]: Define wchar_t to char here, too, since we no
longer include "strlist.h" after the definition in table.c.
* parted/table.c: Include <wchar.h> and <string.h> unconditionally,
and before wchar_t redefinition.
Hoist inclusions of xalloc.h and strlist.h, too.
Remove wcwidth and strnlen declarations, since they're guaranteed
to be in the gnulib-supplied-if-needed headers.