kconfig: use C89 random functions in conf.c

rand and srand functions conform also to C89 in addition to POSIX.1-2001,
which makes them a bit more portable (work also on MinGW host). Linux man
page also says:
"The versions of rand() and srand() in the Linux C Library use the same
random number generator as random() and srandom()".

* Use C89 conformant functions rand() and srand()

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Roman Zippel <zippel@linux-m68k.org>