Add declaration for bpf_skb_adjust_room()

Derived from documentation in //include/uapi/linux/bpf.h

Test: builds
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I6026e7f0429e1d82e0a234b47e261aba60b7d7de
diff --git a/bpf_progs/bpf_net_helpers.h b/bpf_progs/bpf_net_helpers.h
index c4e0b16..3714eac 100644
--- a/bpf_progs/bpf_net_helpers.h
+++ b/bpf_progs/bpf_net_helpers.h
@@ -42,6 +42,9 @@
                                   __u64 flags) = (void*)BPF_FUNC_l4_csum_replace;
 static int (*bpf_redirect)(__u32 ifindex, __u64 flags) = (void*)BPF_FUNC_redirect;
 
+static int (*bpf_skb_adjust_room)(struct __sk_buff* skb, __s32 len_diff, __u32 mode,
+                                  __u64 flags) = (void*)BPF_FUNC_skb_adjust_room;
+
 // Android only supports little endian architectures
 #define htons(x) (__builtin_constant_p(x) ? ___constant_swab16(x) : __builtin_bswap16(x))
 #define htonl(x) (__builtin_constant_p(x) ? ___constant_swab32(x) : __builtin_bswap32(x))