windows-nat.c: Fix bad initialization of ptid

windows-nat.c: Fix bad initialization of ptid

When trying to build for x86_64-w64-mingw32:

/home/simark/src/binutils-gdb/gdb/windows-nat.c: In function ‘void windows_detach(target_ops*, const char*, int)’:
/home/simark/src/binutils-gdb/gdb/windows-nat.c:1915:20: error: converting to ‘ptid_t’ from initializer list would use explicit constructor ‘constexpr ptid_t::ptid_t(int, long int, long int)’
   ptid_t ptid = {-1};
                    ^
Fixed by initializing ptid with the minus_one_ptid variable.

gdb/ChangeLog:

	* windows-nat.c (windows_detach): Initialize ptid with
	minus_one_ptid.