prevent warning noise on mingw-64 builds

The following was produced for every non-miniperl object built,
including for XS modules and the test builds done by
ExtUtils::MakeMaker etc

..\iperlsys.h:640:66: warning: 'struct utimbuf' declared inside parameter list
 typedef int  (*LPLIOUtime)(struct IPerlLIO*, const char*, struct utimbuf*);
                                                                  ^
..\iperlsys.h:640:66: warning: its scope is only this definition or declaration,
 which is probably not what you want

1 files changed, 2 insertions, 0 deletions

diff --git a/iperlsys.h b/iperlsys.h
index 57160e0804..3aee24f7ce 100644
--- a/iperlsys.h
+++ b/iperlsys.h
@@ -600,6 +600,8 @@ struct IPerlEnvInfo
 
 #if defined(PERL_IMPLICIT_SYS)
 
+struct utimbuf; /* prevent gcc warning about the use below */
+
 /* IPerlLIO		*/
 struct IPerlLIO;
 struct IPerlLIOInfo;