Windows: Remove UNIX guard for pstrcmp()

@equalsraf: I took a look at the Vim source pstrcmp() is actually used in the
dos_expandpath(misc1.c). The only difference between the UNIX and WIN32 version
of the functions is the _cdecl call convention annotation - the body of the
function is identical. Neovim kept the comment from the Unix function but not
from the Windows variant. Seems to me its safe to use the same function for
both - and just correct the comment.

src/nvim/path.c

@@ -467,16 +467,13 @@ bool path_has_wildcard(const char_u *p)
   return false;
 }
 
-#if defined(UNIX)
 /*
  * Unix style wildcard expansion code.
- * It's here because it's used both for Unix and Mac.
  */
 static int pstrcmp(const void *a, const void *b)
 {
   return pathcmp(*(char **)a, *(char **)b, -1);
 }
-#endif
 
 /// Checks if a path has a character path_expand can expand.
 /// @param p  The path to expand.