libbpf: Install btf.h with libbpf

install_headers target should contain all headers that are part of
libbpf. Add missing btf.h

Signed-off-by: Andrey Ignatov <[e-mail]>
Acked-by: Martin KaFai Lau <[e-mail]>
Acked-by: Song Liu <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>

tools/lib/bpf/Makefile

···
 	$(call QUIET_INSTALL, headers) \
 		$(call do_install,bpf.h,$(prefix)/include/bpf,644); \
 		$(call do_install,libbpf.h,$(prefix)/include/bpf,644);
+		$(call do_install,btf.h,$(prefix)/include/bpf,644);
 
 install: install_lib