Add LIBBPF_VERSION

LIBBPF_VERSION is already used to generate libbpf.pc file but was not
actuall set.

Set it same way it's done in kernel tree. Version is in sync with
version script for DSO.

Before:
  % grep Version src/libbpf.pc
  Version:

After:
  % grep Version src/libbpf.pc
  Version: 0.0.2

Fixes: 93bc1d0 ("makefile: sync generate pkg-config file for libbpf")
Signed-off-by: Andrey Ignatov <rdna@fb.com>

src/Makefile

@@ -1,5 +1,11 @@
 # SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
 
+VERSION = 0
+PATCHLEVEL = 0
+EXTRAVERSION = 2
+
+LIBBPF_VERSION	= $(VERSION).$(PATCHLEVEL).$(EXTRAVERSION)
+
 TOPDIR = ..
 
 INCLUDES := -I. -I$(TOPDIR)/include -I$(TOPDIR)/include/uapi