ktrace: Only enable EXTERROR traces for 'x'

Add a missing break statement to avoid falling through to the '+'
case.

Fixes:		96f4be881e ("ktrace(1): teach about KTR_EXTERR")

usr.bin/ktrace/subr.c

@@ -89,6 +89,7 @@ getpoints(char *s)
 			break;
 		case 'x':
 			facs |= KTRFAC_EXTERR;
+			break;
 		case '+':
 			facs |= DEF_POINTS;
 			break;