gdbserver: fix format string warning in win32-low.cc

gdbserver: fix format string warning in win32-low.cc

When compiling on Cygwin, we get:

      CXX    win32-low.o
    /home/smarchi/src/binutils-gdb/gdbserver/win32-low.cc: In function ‘int get_child_debug_event(DWORD*, target_waitstatus*)’:
    /home/smarchi/src/binutils-gdb/gdbserver/win32-low.cc:1459:17: error: format ‘%x’ expects argument of type ‘unsigned int’, but argument 2 has type ‘long int’ [-Werror=format=]
     1459 |       OUTMSG2 (("get_windows_debug_event - "
          |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
     1460 |   "unexpected stop in 0x%x (expecting 0x%x)\n",
          |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     1461 |   ptid.lwp (), desired_stop_thread_id));
          |   ~~~~~~~~~~~
          |            |
          |            long int
    /home/smarchi/src/binutils-gdb/gdbserver/win32-low.cc:52:11: note: in definition of macro ‘OUTMSG2’
       52 |    printf X;    \
          |           ^
    /home/smarchi/src/binutils-gdb/gdbserver/win32-low.cc:1460:26: note: format string is defined here
     1460 |   "unexpected stop in 0x%x (expecting 0x%x)\n",
          |                         ~^
          |                          |
          |                          unsigned int
          |                         %lx

`ptid.lwp ()` is a `long` value, so it indeed needs the `l` size modifier.

gdbserver/ChangeLog:

	* win32-low.cc (get_child_debug_event): Fix format string warning.