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

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

This is similar to 3c3d384ae93700ef08545b078c37065fdb98eee7 and
a workaround for the following warning.
```
In file included from ../src/basic/in-addr-util.h:28,
                 from ../src/nss-mymachines/nss-mymachines.c:31:
../src/nss-mymachines/nss-mymachines.c: In function '_nss_mymachines_getgrnam_r':
../src/nss-mymachines/nss-mymachines.c:653:32: 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)))
                                       ^
../src/nss-mymachines/nss-mymachines.c: In function '_nss_mymachines_getgrgid_r':
../src/nss-mymachines/nss-mymachines.c:730:32: 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)))
                                       ^
```