Merge #15782: Avoid redefine warning

0b3a654 Avoid redefine warning (Peter Bushnell)

Pull request description:

  Wrap preprocessor definition of NOMINMAX in ifndef conditional to suppress warning when cross compiling Windows.

  `fs.cpp:6:0: warning: "NOMINMAX" redefined`
  `/usr/lib/gcc/x86_64-w64-mingw32/7.3-posix/include/c++/x86_64-w64-mingw32/bits/os_defines.h:45:0: note: this is the location of the previous definition
   #define NOMINMAX 1`

  #define NOMINMAX was introduced in the following merge.

  bitcoin/bitcoin#14426

ACKs for commit 0b3a65:
  practicalswift:
    utACK 0b3a654
  promag:
    utACK 0b3a654.

Tree-SHA512: 0175195b88e63d3d44ffac2b8cc87ae7b285a45ed4e49605bca0cc82db073006c22024ef9c2f287980d357dac1099f798f1eeaa0bd75bb7a625919dc1632366c

Author: laanwj

src/fs.cpp

@@ -3,7 +3,9 @@
 #ifndef WIN32
 #include <fcntl.h>
 #else
+#ifndef NOMINMAX
 #define NOMINMAX
+#endif
 #include <codecvt>
 #include <windows.h>
 #endif