Add a fall-through comment to stabsread.c

Add a fall-through comment to stabsread.c

This adds a fall-through comment to stabsread.c.  I skimmed the stabs
manual a bit and it seems that 'p' and 'P' are similar enough that
this makes sense.  Also, stabs is mostly deprecated, and the code has
been this way for a long time, so it seemed safest to keep the status
quo.

ChangeLog
2018-05-04  Tom Tromey  <tom@tromey.com>

	* stabsread.c (define_symbol) <case 'p'>: Add fall-through
	comment.