libbpf: Makefile set specified permission mode

The third parameter to do_install was not used by $(INSTALL) command.
Fix this by only setting the -m option when the third parameter is supplied.

The use of a third parameter was introduced in commit  eb54e522 ("bpf:
install libbpf headers on 'make install'").

Without this change, the header files are install as executables files (755).

Fixes: eb54e522 ("bpf: install libbpf headers on 'make install'")
Signed-off-by: NJesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

tools/lib/bpf/Makefile

@@ -172,7 +172,7 @@ define do_install
 	if [ ! -d '$(DESTDIR_SQ)$2' ]; then		\
 		$(INSTALL) -d -m 755 '$(DESTDIR_SQ)$2';	\
 	fi;						\
-	$(INSTALL) $1 '$(DESTDIR_SQ)$2'
+	$(INSTALL) $1 $(if $3,-m $3,) '$(DESTDIR_SQ)$2'
 endef
 
 install_lib: all_cmd