Include memmodel.h in adjust-alignment.c

Commit dfa4fcdb broke SPARC bootstrap:

In file included from ./tm_p.h:4,
                 from /vol/gcc/src/hg/master/local/gcc/adjust-alignment.c:28:
/vol/gcc/src/hg/master/local/gcc/config/sparc/sparc-protos.h:45:47: error: use of enum 'memmodel' without previous declaration
 extern void sparc_emit_membar_for_model (enum memmodel, int, int);
                                               ^~~~~~~~

Fixed by including memmodel.h.  Bootstrapped on sparc-sun-solaris2.11
and i386-pc-solaris2.11.

2020-05-21  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	gcc/
	* adjust-alignment.c: Include memmodel.h.