fix(xml): guard NOMINMAX so a consumer that already defines it still builds

XmlReader.cpp defined WIN32_LEAN_AND_MEAN and NOMINMAX unconditionally before
including <windows.h>. CNA compiles the whole tree with -DNOMINMAX on the
command line and with /W4 /WX, so MSVC reported the redefinition as C4005 and
/WX turned it into C2220 -- the CNA + sharp-runtime pair did not build at all
on Windows at these two HEADs.

Every other Windows translation unit here already uses the #ifndef guard,
Environment.cpp from this same series included; this file was the one that
missed it.

Linux is unaffected: the block is inside #if defined(_WIN32).

Author: robertvokac

modules/xml/src/System/Xml/XmlReader.cpp

@@ -8,8 +8,12 @@
 #if defined(_WIN32)
 // Contained in this translation unit: <windows.h> in a header is what produced the ERROR and
 // min/max macro collisions recorded in CNA as WINNATIVE-F5 and F11.
-#  define WIN32_LEAN_AND_MEAN
-#  define NOMINMAX
+#  ifndef WIN32_LEAN_AND_MEAN
+#    define WIN32_LEAN_AND_MEAN
+#  endif
+#  ifndef NOMINMAX
+#    define NOMINMAX
+#  endif
 #  include <windows.h>
 #endif
 #include <algorithm>