pcap-bpf: Fix a 'not defined' macro error

The error was:
./pcap-bpf.c: In function 'pcap_activate_bpf':
./pcap-bpf.c:2334:5: error: "_BSDI_VERSION" is not defined, evaluates
to 0 [-Werror=undef]
 2329 | #if _BSDI_VERSION - 0 >= 199510
      |     ^~~~~~~~~~~~~

1 files changed, 1 insertions, 1 deletions

diff --git a/pcap-bpf.c b/pcap-bpf.c
index 819ec7ed..16d33501 100644
--- a/pcap-bpf.c
+++ b/pcap-bpf.c
@@ -2326,7 +2326,7 @@ pcap_activate_bpf(pcap_t *p)
 		goto bad;
 	}
 #endif
-#if _BSDI_VERSION - 0 >= 199510
+#if defined(_BSDI_VERSION) && _BSDI_VERSION >= 199510
 	/* The SLIP and PPP link layer header changed in BSD/OS 2.1 */
 	switch (v) {