[PATCH][PR build/23568] Fix gdb-7.12.1 MinGW/MSYS build issue

[SiZiOUS]

This little patch was made to allow the compilation under the MinGW/MSYS legacy environment (I don't know for the newer MinGW-w64/MSYS2 environment!).
If this patch is not applied, then the compilation of gdb-7.12.1 under MinGW/MSYS is impossible.

This fix is not intrusive as it's taken into account only if ERROR_IO_PENDING is not declared in the current scope, so it should be considered safe.
See the Bugzilla bug entry for more details.

Please note, this fix only allows the build possible under MinGW/MSYS, there is no bug fixes or improvements for GDB itself.

gdb/ChangeLog:
2018-08-28  SiZiOUS<sizious@gmail.com>

	PR build/23568
	* ser-mingw.c: Conditioned fix allowing compilation under
	the MinGW/MSYS legacy environment ('ERROR_IO_PENDING' was not
	declared in the scope).
  

-------------- next part --------------
--- ser-mingw.c	2017-01-21 13:46:47 +0000
+++ ser-mingw.c-pr_build_23568	2018-08-28 11:44:38 +0000
@@ -31,6 +31,10 @@
 
 #include "command.h"
 
+#ifndef ERROR_IO_PENDING
+#include <winerror.h>
+#endif
+
 void _initialize_ser_windows (void);
 
 struct ser_windows_state