troff: we are not htmlroff

If we don’t explicitly check for ‘h’ in troff, we can’t reliably check
for non-htmlroff well.

Consider the following:

	.if h \{\
	.	de M
	.		tm m
	..\}

Without this change, this will print m and not define macro M.

sys/src/cmd/troff/n5.c

@@ -683,6 +683,7 @@ void caseif1(int x)
 		if (TROFF)
 			true++;
 		break;
+	case 'h':
 	case ' ':
 		break;
 	default: