compat/arc4random.c: use memset instead of explicit_bzero (#252)

compat/arc4random.c: use memset instead of explicit_bzero (#252)

Use memset instead of explicit_bzero to avoid the following build
failure with uclibc-ng since version 10.0.3 and
https://github.com/NetworkConfiguration/dhcpcd/commit/837d09e34c487edaa92aa2ae71a630d84c927f8e:

/home/fabrice/buildroot/output/host/lib/gcc/arm-buildroot-linux-uclibcgnueabi/12.3.0/../../../../arm-buildroot-linux-uclibcgnueabi/bin/ld: ../compat/arc4random.o: in function `_rs_stir_if_needed':
arc4random.c:(.text+0x8cc): undefined reference to `explicit_bzero'

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>