Conditionally include "<windows.h>" on common/pathstuff.c (and unbreak build on mingw*)

Conditionally include "<windows.h>" on common/pathstuff.c (and unbreak build on mingw*)

commit b4987c956dfa44ca9fd8552f63e15f5fa094b2a4
Author: Sergio Durigan Junior <sergiodj@redhat.com>
Date:   Fri Feb 9 18:44:59 2018 -0500

    Create new common/pathstuff.[ch]

Introduced a regression when compiling for mingw*:

  /gdb/common/pathstuff.c: In function 'gdb::unique_xmalloc_ptr<char>
  gdb_realpath(const char*)':
  /gdb/common/pathstuff.c:56:14: error: 'MAX_PATH' was not declared in this scope
       char buf[MAX_PATH];
		^
  /gdb/common/pathstuff.c:57:5: error: 'DWORD' was not declared in this scope
       DWORD len = GetFullPathName (filename, MAX_PATH, buf, NULL);
       ^
  /gdb/common/pathstuff.c:57:11: error: expected ';' before 'len'
       DWORD len = GetFullPathName (filename, MAX_PATH, buf, NULL);
	     ^
  /gdb/common/pathstuff.c:63:9: error: 'len' was not declared in this scope
       if (len > 0 && len < MAX_PATH)
	   ^
  /gdb/common/pathstuff.c:64:54: error: 'buf' was not declared in this scope
	 return gdb::unique_xmalloc_ptr<char> (xstrdup (buf));
							^
  make[2]: *** [pathstuff.o] Error 1

The proper fix is to conditionally include "<windows.h>".  This commit
does that, without introducing any regressions as per tests made by
our BuildBot.

gdb/ChangeLog:
2018-03-01  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR gdb/22907
	* common/pathstuff.c: Conditionally include "<windows.h>".