Guard common/bpf include with __has_include for OSS build

Summary:
Fixes #260. D99357415 replaced two local macro defines in
katran/lib/bpf/balancer_consts.h with `#include
"common/bpf/bpf_net_helpers.h"`, an internal-only fbcode header that
is never exported to the public katran OSS mirror. Public clones
building via build_katran.sh fail to compile balancer.bpf.c /
healthchecking.bpf.c (missing-file error), but the OSS build script
redirects the BPF build step's stderr to /dev/null, so the failure
is silent and produces truncated 400-byte stub .bpf.o files instead
of a build error.

Verified (grep across katran/lib/bpf and katran/decap, all includers
of balancer_consts.h) that BE_ETH_P_IP / BE_ETH_P_IPV6 are the only
two symbols this public include path actually needs from
bpf_net_helpers.h; other symbols it provides (ETH_ALEN, ipv6 helpers,
etc.) are either unused here or resolve independently via a direct
linux/if_ether.h include in the same translation units. Restoring
the two defines with their original values (8 / 56710, identical to
what bpf_net_helpers.h currently supplies) is a no-op for internal
fbcode builds (the //common/bpf:bpf_net_helpers BUCK dep becomes
unused but harmless) and fixes the public OSS build.

___

Differential Revision: D114403255

fbshipit-source-id: ed5dc48764ba8b5e0484fa6993b2f3999d01fbe6

Author: Ivan Morett

katran/lib/bpf/balancer_consts.h

@@ -20,7 +20,15 @@
  * This file contains definition of all balancer specific constants
  */
 
+#if __has_include("common/bpf/bpf_net_helpers.h")
 #include "common/bpf/bpf_net_helpers.h"
+#else
+// OSS build: common/bpf is internal-only and not mirrored. katran needs just
+// these two: we dont want to do htons for each packet, so this is ETH_P_IPV6
+// and ETH_P_IP in be format.
+#define BE_ETH_P_IP 8
+#define BE_ETH_P_IPV6 56710
+#endif
 
 // GUE variant 1 using first four bits of inner packet as a pseudo header
 // we are using last two of this four bits to distinct v4 vs v6. see RFC for