[sim] Include defs.h in ppc/hw_memory.c

[sim] Include defs.h in ppc/hw_memory.c

To fix this error (seen on cygwin):
/../../sim/ppc/../common ../../../sim/ppc/hw_memory.c
In file included from ../../gnulib/import/stdlib.h:100,
                 from ../../../sim/ppc/hw_memory.c:28:
../../gnulib/import/unistd.h:663:3: error: #error "Please include config.h first."
  663 |  #error "Please include config.h first."
      |   ^~~~~
../../gnulib/import/unistd.h:665:24: error: expected ';' before 'extern'
  665 | _GL_INLINE_HEADER_BEGIN
      |                        ^
      |                        ;
../../gnulib/import/unistd.h:2806:22: error: expected ';' before 'extern'
 2806 | _GL_INLINE_HEADER_END
      |                      ^
      |                      ;