common.mk: Add /usr/include/x86_64-linux-gnu to include path

Seems that on Ubuntu some of the kernel headers are in that directory, and
that it is not in the include path by default.

Fixes one of the issues in #44.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>

Author: tohojo

common/common.mk

@@ -38,7 +38,7 @@ EXTRA_DEPS +=
 # BPF-prog kern and userspace shares struct via header file:
 KERN_USER_H ?= $(wildcard common_kern_user.h)
 
-CFLAGS ?= -I$(LIBBPF_DIR)/root/usr/include/
+CFLAGS ?= -I$(LIBBPF_DIR)/root/usr/include/ -I/usr/include/x86_64-linux-gnu
 CFLAGS += -I../headers/
 LDFLAGS ?= -L$(LIBBPF_DIR)