exec: Create include for target_page_size()

That is the only function that we need from exec.c, and having to
include the whole sysemu.h for this.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

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/me leans to be less sloppy with copyright notices
thanks Dave