[gdbserver] Fix Wlto-type-mismatch for debug_agent

When building gdb including gdbserver with CFLAGS/CXXFLAGS -O2 -g -flto=auto,
I run into:
...
src/gdbserver/../gdbsupport/agent.h:47:13: error: type of 'debug_agent' \
  does not match original declaration [-Werror=lto-type-mismatch]
 extern bool debug_agent;
             ^
src/gdbserver/ax.cc:28:5: note: type 'int' should match type 'bool'
 int debug_agent = 0;
     ^
src/gdbserver/ax.cc:28:5: note: 'debug_agent' was previously declared here
src/gdbserver/ax.cc:28:5: note: code may be misoptimized unless \
  -fno-strict-aliasing is used
...

Fix this by changing the type of debug_agent in ax.cc from int to bool.

Tested on x86_64-linux.