hurd: fix gnu_debug_flag type

Fixes

../../gdb/gnu-nat.c:96:6: error: conflicting declaration ‘bool gnu_debug_flag’
   96 | bool gnu_debug_flag = false;
../../gdb/gnu-nat.c: In function ‘void _initialize_gnu_nat()’:
../../gdb/gnu-nat.c:3511:7: error: cannot

gdb/ChangeLog:

	* gnu-nat.h (gnu_debug_flag): Set type to bool.