[gdbserver] Add missing include of gdbsupport/agent.h

The file gdbserver/ax.h contains:
...
 #ifdef IN_PROCESS_AGENT
 #define debug_threads debug_agent
 #endif
...
but does not declare debug_agent.

Fix this by adding an include of gdbsupport/agent.h.

[ If this fix would have been in place before commit 8118159c "[gdbserver] Fix
Wlto-type-mismatch for debug_agent", we would have simply run into this build
breaker with a regular, non-lto build:
...
src/gdbserver/ax.cc:28:5: error: conflicting declaration 'int debug_agent'
 int debug_agent = 0;
     ^~~~~~~~~~~
In file included from src/gdbserver/ax.h:25:0,
                 from src/gdbserver/ax.cc:20:
src/gdbsupport/agent.h:47:13: note: previous declaration as 'bool debug_agent'
 extern bool debug_agent;
             ^~~~~~~~~~~
... ]

Tested on x86_64-linux.

gdbserver/ChangeLog:

2020-06-29  Tom de Vries  <tdevries@suse.de>

	* ax.h: Include gdbsupport/debug_agent.h.