[PATCH] Fix build warning in mro.c

Attached patch fixes the following build warning introduced
by commit 73519bd01829f1480c288a0a7ccbfff973d867df:

mro.c: In function `S_mro_get_linear_isa_dfs':
mro.c:327: warning: passing arg 2 of `Perl_sv_2mortal' from
incompatible pointer type

• [PATCH] Fix build warning in mro.c by Jerry D. Hedden
• Re: [PATCH] Fix build warning in mro.c by Nicholas Clark