Constify annotate_source

I noticed that annotate_source takes a "char *", but really should
take a "const char *".  This patch fixes this.

gdb/ChangeLog
2019-05-17  Tom Tromey  <tromey@adacore.com>

	* annotate.c (annotate_source): Make "filename" const.
	* annotate.h (annotate_source): Use const.
3 files changed