build(windows): guard NOMINMAX before including windows.h

Four sources defined NOMINMAX unconditionally, which redefines the macro the build already passes and warns on every MinGW build. Use the #ifndef guard the rest of the tree uses.

core/libs/prx/libc/src/Filesystem.cpp

@@ -2,7 +2,9 @@
 #include <cerrno>
 #include <vector>
 #ifdef _WIN32
+#ifndef NOMINMAX
 #define NOMINMAX
+#endif
 #include <windows.h>
 #else
 #include <unistd.h>

core/libs/prx/libkernel/DirectMemory/Export.cpp

@@ -7,7 +7,9 @@
 #include "prx/libkernel/DirectMemory/DirectMemory.hpp"
 #ifdef _WIN32
 #define WIN32_LEAN_AND_MEAN
+#ifndef NOMINMAX
 #define NOMINMAX
+#endif
 #include <windows.h>
 #else
 #include <cerrno>

core/libs/prx/libkernel/Module/src/DynamicLoader.cpp

@@ -9,7 +9,9 @@
 #include <string>
 #include <vector>
 #ifdef _WIN32
+#ifndef NOMINMAX
 #define NOMINMAX
+#endif
 #include <windows.h>
 #else
 #include <dlfcn.h>

core/libs/tests/GuestAccess.cpp

@@ -3,7 +3,9 @@
 #include <cstdlib>
 #include <fstream>
 #ifdef _WIN32
+#ifndef NOMINMAX
 #define NOMINMAX
+#endif
 #include <windows.h>
 #include <aclapi.h>
 #endif