libbpf: Install headers as part of make install

Current 'make install' results in only pkg-config and library binaries
being installed. For consistency also install headers as part of
"make install"
Signed-off-by: NNikolay Borisov <nborisov@suse.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Acked-by: NAndrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200526174612.5447-1-nborisov@suse.comSigned-off-by: NAlexei Starovoitov <ast@kernel.org>

tools/lib/bpf/Makefile

@@ -264,7 +264,7 @@ install_pkgconfig: $(PC_FILE)
 	$(call QUIET_INSTALL, $(PC_FILE)) \
 		$(call do_install,$(PC_FILE),$(libdir_SQ)/pkgconfig,644)
 
-install: install_lib install_pkgconfig
+install: install_lib install_pkgconfig install_headers
 
 ### Cleaning rules