Two minor addrmap cleanups

While working on a different patch, I found a couple of simple addrmap
cleanups.

In one case, a forward declaration is no longer needed, as the header
now includes addrmap.h.

In the other, an include of addrmap.h is no longer needed.

Tested by rebuilding.

gdb/buildsym.h

@@ -25,7 +25,6 @@
 
 struct objfile;
 struct symbol;
-struct addrmap;
 struct compunit_symtab;
 enum language;
 

gdb/psymtab.c

@@ -24,7 +24,6 @@
 #include "block.h"
 #include "filenames.h"
 #include "source.h"
-#include "addrmap.h"
 #include "gdbtypes.h"
 #include "ui-out.h"
 #include "command.h"