base: Fix unused function warning

After refactoring the remote gdb interface, break_type is declared as
const function and is only used as a parameter to DPRINTF function
calls. This means that it is seen as unused when compiling
gem5.fast. This changeset fixes the warning.

Change-Id: Iea89b66c53c62341c043d8bd3838ebc27ee333bc
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/7741
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>

src/base/remote_gdb.cc

@@ -290,6 +290,7 @@ enum GdbBreakpointType {
     GdbAccWp = '4',
 };
 
+#ifndef NDEBUG
 const char *
 break_type(char c)
 {
@@ -302,6 +303,7 @@ break_type(char c)
       default: return "unknown breakpoint/watchpoint";
     }
 }
+#endif
 
 std::map<Addr, HardBreakpoint *> hardBreakMap;