pdp11-aout: Remove invalid conversion and restore build

When cross compiling to pdp11-aout, the build has been recently
failing with

  error: invalid conversion from 'int*' to 'unsigned int*'

This patch changes the variable in question to the appropriate type
and fixes the issue (I have not looked at the code beyond that).

Tested by successfully configuring gcc with

  ../src/configure --prefix=/home/mjambor/gcc/mine/inst --enable-languages=c,c++ --enable-checking=yes --disable-bootstrap --disable-multilib --enable-obsolete --target=pdp11-aout

and running make all-host.

gcc/ChangeLog:

2026-06-09  Martin Jambor  <mjambor@suse.cz>

	* config/pdp11/pdp11.cc (pdp11_conditional_register_usage): Change i
	from int to unsigned int.