Fix warning: "HAVE_SCTP" redefined (#1010)

The netinet/sctp.h header file defines HAVE_SCTP, so we have this
warning while building:

    In file included from ../../src/iperf.h:30,
                     from ../../src/iperf_sctp.c:46:
    ./iperf_config.h:44: warning: "HAVE_SCTP" redefined
       44 | #define HAVE_SCTP 1
          |
    In file included from ../../src/iperf_sctp.c:43:
    /usr/include/netinet/sctp.h:56: note: this is the location of the previous definition
       56 | #define HAVE_SCTP
          |

This patch replaces our HAVE_SCTP definition with HAVE_SCTP_H to avoid
this warning.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
diff --git a/configure b/configure
index f0b0db8..eff3888 100755
--- a/configure
+++ b/configure
@@ -13416,7 +13416,7 @@
 #define HAVE_NETINET_SCTP_H 1
 _ACEOF
 
-$as_echo "#define HAVE_SCTP 1" >>confdefs.h
+$as_echo "#define HAVE_SCTP_H 1" >>confdefs.h
 
 		 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for library containing sctp_bindx" >&5
 $as_echo_n "checking for library containing sctp_bindx... " >&6; }