Make _SNETVERSIONDATA char* fields const

Follow-up to https://github.com/diasurgical/devilution/commit/ec329e645c4794b63a8239f6589260ed8a7c0b2

structs.h

@@ -1428,10 +1428,10 @@ typedef struct _SNETUIDATA {
 
 typedef struct _SNETVERSIONDATA {
 	int size;
-	char *versionstring;
-	char *executablefile;
-	char *originalarchivefile;
-	char *patcharchivefile;
+	const char *versionstring;
+	const char *executablefile;
+	const char *originalarchivefile;
+	const char *patcharchivefile;
 } _SNETVERSIONDATA;
 
 // TPDEF PTR FCN UCHAR SNETSPIBIND