Fix error message in compile-object-load.c

Fix error message in compile-object-load.c

I noticed that an error message in compile-object-load.c mentions the
wrong symbol name.  The loop just above the error is looking for
COMPILE_I_EXPR_VAL, but the error references COMPILE_I_EXPR_PTR_TYPE.

I'm checking this in as obvious.  I don't have a test case -- I
noticed it because another patch I'm working on caused this error to
be thrown, but that was due to regression in my patch.

gdb/ChangeLog
2020-03-25  Tom Tromey  <tom@tromey.com>

	* compile/compile-object-load.c (get_out_value_type): Mention
	correct symbol name in error message.