fdisk, sfdisk: avoid non-ANSI function declarations [smatch scan]

fdisk, sfdisk: avoid non-ANSI function declarations [smatch scan]

Adding _FUNCTION_DEF definition will exclude compatibility type definitions
that do include void key word in empty argument list.

/usr/include/readline/rltypedefs.h:35:23: warning: non-ANSI function
declaration of function 'Function'

These functions has been replaced by set of new ones in readline 4.2 (April
2001), and removed in 6.3 (February 2014).

Reference: https://cnswww.cns.cwru.edu/php/chet/readline/CHANGES
Rererence: https://blueslugs.com/blog/2016-10-23-updating-cppfunction-in-old-readline-consumers/
Signed-off-by: Sami Kerola <kerolasa@iki.fi>