In common.mk define LIBBPF_DIR if Makefile including didn't

This was an easy solution to the worry in commit 064365f
that Makefile including common.mk could forget to define LIBBPF_DIR.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>

Author: netoptimizer

common/common.mk

@@ -21,6 +21,7 @@ USER_OBJ := ${USER_C:.c=.o}
 
 # Expect this is defined by including Makefile, but define if not
 COMMON_DIR ?= ../common/
+LIBBPF_DIR ?= ../libbpf/src/
 
 COPY_LOADER ?=
 LOADER_DIR ?= $(COMMON_DIR)/../basic-solutions