misc: fix compilation warnings

Introduced in previous commit:

../src/misc.c: In function ‘fork_exec’: ../src/misc.c:184:9: warning: assignment to ‘GError *’ {aka ‘struct _GError *’} from ‘gboolean’ {aka ‘int’} makes pointer from integer without a cast [-Wint-conversion] 184 | err = g_shell_parse_argv(ex, &argc, &argv, &err); | ^ ../src/misc.c:173:14: warning: unused variable ‘ret’ [-Wunused-variable] 173 | gboolean ret; | ^~~

Signed-off-by: Marc-André Lureau marcandre.lureau@redhat.com