system/Error: use std::generic_category() for errno on Windows

It's wrong to use std::system_category() for both GetLastError() and
errno on Windows.  Apparently, everybody uses std::generic_category()
for errno values, which appears to be a safe choice.

Some discussion on this confusing topic can be found here:

 https://stackoverflow.com/questions/28746372/system-error-categories-and-standard-system-error-codes

src/decoder/plugins/FlacIOHandle.cxx

@@ -21,8 +21,7 @@
 #include "FlacIOHandle.hxx"
 #include "Log.hxx"
 #include "Compiler.h"
-
-#include <system_error>
+#include "system/Error.hxx"
 
 #include <errno.h>
 #include <stdio.h>
@@ -49,7 +48,7 @@ FlacIORead(void *ptr, size_t size, size_t nmemb, FLAC__IOHandle handle)
 
 #ifndef WIN32
 		} catch (const std::system_error &e) {
-			errno = e.code().category() == std::system_category()
+			errno = e.code().category() == ErrnoCategory()
 				? e.code().value()
 				/* just some random non-zero errno
 				   value */