gdb: add forward declarations in maint.h

Editing maint.h with clangd shows some errors about obj_section and
objfile being unknown.  Add some forward declarations for them.

Change-Id: Ic4dd12a371198fdf740892254a8f2c1fae2846b9

gdb/maint.h

diff --git a/gdb/maint.h b/gdb/maint.h
index 434d2a9..0ddc62b 100644
--- a/gdb/maint.h
+++ b/gdb/maint.h
@@ -22,6 +22,9 @@
 #include "gdbsupport/run-time-clock.h"
 #include <chrono>
 
+struct obj_section;
+struct objfile;
+
 extern void set_per_command_time (int);
 
 extern void set_per_command_space (int);