Sometimes obj directory was not created during
parallel builds

Related bugreports:
https://bugs.gentoo.org/880051
https://bugs.gentoo.org/911984

--- a/Makefile	2023-08-10 11:43:39.826589819 +0300
+++ b/Makefile	2023-08-10 11:43:50.121722774 +0300
@@ -90,6 +90,7 @@
 	touch $@
 
 obj/%.o: src/%.c
+	mkdir -p obj
 	$(CC) $(CFLAGS) -c $< -o $@
 
 clean: