backends: Don't depend on linux/bpf.h to compile bpf disassembler.

We only need a few constants and one structure definition from linux/bpf.
Just define those in a local lib/bpf.h file. This makes sure the bpf
disassembler is always build and included even when elfutils is build
on older GNU/Linux systems (and even on other platforms).

Signed-off-by: Mark Wielaard <mark@klomp.org>
diff --git a/lib/Makefile.am b/lib/Makefile.am
index 7a65eb9..ada2030 100644
--- a/lib/Makefile.am
+++ b/lib/Makefile.am
@@ -38,7 +38,7 @@
 		  color.c printversion.c
 
 noinst_HEADERS = fixedsizehash.h libeu.h system.h dynamicsizehash.h list.h \
-		 md5.h sha1.h eu-config.h color.h printversion.h
+		 md5.h sha1.h eu-config.h color.h printversion.h bpf.h
 EXTRA_DIST = dynamicsizehash.c
 
 if !GPROF