Add a drake_vendor namespace ala tools/workspace/vendor_cxx

This prevents link-time symbol conflicts in case code downstream of
Drake wants to use a different build of tinyxml2.

Note that the upstream source repository contains windows newlines
throughout, so we need to entertain `^M` shrapnel throughout this
patch file.

--- tinyxml2.h.orig	2021-06-06 17:10:24.000000000 -0700
+++ tinyxml2.h	2022-09-03 07:30:36.319764029 -0700
@@ -42,6 +42,8 @@
 #endif
 #include <stdint.h>
 
+inline namespace drake_vendor __attribute__ ((visibility ("hidden"))) {
+
 /*
    TODO: intern strings instead of allocation.
 */
@@ -2377,4 +2379,6 @@
 #   pragma warning(pop)
 #endif
 
+}  // namespace drake_vendor
+
 #endif // TINYXML2_INCLUDED
--- tinyxml2.cpp.orig	2021-06-06 17:10:24.000000000 -0700
+++ tinyxml2.cpp	2022-09-03 07:31:12.036248705 -0700
@@ -32,6 +32,8 @@
 #   include <cstdarg>
 #endif
 
+inline namespace drake_vendor __attribute__ ((visibility ("hidden"))) {
+
 #if defined(_MSC_VER) && (_MSC_VER >= 1400 ) && (!defined WINCE)
 	// Microsoft Visual Studio, version 2005 and higher. Not WinCE.
 	/*int _snprintf_s(
@@ -2984,3 +2986,5 @@
 }
 
 }   // namespace tinyxml2
+
+}  // namespace drake_vendor
