bpf: install libbpf headers on 'make install'

Add a new target to install the bpf.h header to $(prefix)/include/bpf/
directory.  This is necessary to build standalone applications using
libbpf, without the need to clone the kernel sources and point to them.
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/lib/bpf/Makefile

@@ -189,6 +189,10 @@ install_lib: all_cmd
 	$(call QUIET_INSTALL, $(LIB_FILE)) \
 		$(call do_install,$(LIB_FILE),$(libdir_SQ))
 
+install_headers:
+	$(call QUIET_INSTALL, headers) \
+		$(call do_install,bpf.h,$(prefix)/include/bpf,644)
+
 install: install_lib
 
 ### Cleaning rules