nss-systemd: add work-around to silence gcc warning

nss-systemd: add work-around to silence gcc warning

In file included from ../src/basic/fs-util.h:32,
                 from ../src/nss-systemd/nss-systemd.c:28:
../src/nss-systemd/nss-systemd.c: In function '_nss_systemd_getgrnam_r':
../src/nss-systemd/nss-systemd.c:416:32: warning: argument to 'sizeof' in 'memset' call is the same pointer type 'char *' as the destination; expected 'char' or an explicit length [-Wsizeof-pointer-memaccess]
         memzero(buffer, sizeof(char*));
                                ^~~~
../src/basic/util.h:118:39: note: in definition of macro 'memzero'
 #define memzero(x,l) (memset((x), 0, (l)))
                                       ^

gcc is trying to be helpful, and it's not far from being right. It _looks_ like
sizeof(char*) is an error, but in this case we're really leaving a space empty
for a pointer, and our calculation is correct. Since this is a short file,
let's just use simplest option and turn off the warning above the two functions
that trigger it.