Add XML parser library

Fork xml.c because of filename issues in Windows.

.gitmodules

@@ -43,3 +43,6 @@
 [submodule "subprojects/SDL_GameControllerDB"]
 	path = subprojects/SDL_GameControllerDB
 	url = https://github.com/gabomdq/SDL_GameControllerDB
+[submodule "subprojects/xml-c"]
+	path = subprojects/xml-c
+	url = https://code.modal.sh/TheoryOfNekomata/xml-c.git

subprojects/xml-c

@@ -0,0 +1 @@
+Subproject commit 02b98a5d98f6f3d19d3edca8634ef87b21a8a256