Apply tentative fix for clang 11 warning in pam_exec(8):

lib/libpam/modules/pam_exec/pam_exec.c:222:56: error: format specifies type 'char *' but the argument has type 'const void *' [-Werror,-Wformat]

if (asprintf(&envstr, "%s=%s", pam_item_env[i].name, item) < 0)
                          ~~                         ^~~~