src/su.c: Fix type of variable

src/su.c: Fix type of variable

su.c:678:26: warning: format ‘%s’ expects argument of type ‘char *’, but argument 4 has type ‘const void *’ [-Wformat=]
su.c:681:44: warning: format ‘%s’ expects argument of type ‘char *’, but argument 3 has type ‘const void *’ [-Wformat=]
su.c:683:46: warning: format ‘%s’ expects argument of type ‘char *’, but argument 3 has type ‘const void *’ [-Wformat=]

Reported-by: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>