add qssb.h submodule

.gitmodules

@@ -4,3 +4,6 @@
 [submodule "submodules/cpp-httplib"]
 	path = submodules/cpp-httplib
 	url = https://github.com/yhirose/cpp-httplib
+[submodule "submodules/qssb.h"]
+	path = submodules/qssb.h
+	url = https://git.quitesimple.org/qssb.h