make this repo depend on the up2date xor_singleheader lib

via a gitsubmodule
at https://github.com/FastFilter/xor_singleheader.git

rather than an older and local copy of that lib

Author: oschonrock

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "dependencies/fastfilter_cpp"]
 	path = dependencies/fastfilter_cpp
 	url = https://github.com/FastFilter/fastfilter_cpp.git
+[submodule "dependencies/xor_singleheader"]
+	path = dependencies/xor_singleheader
+	url = https://github.com/FastFilter/xor_singleheader.git