build: add -Wswitch to clang for more consistency with gcc

gcc12 and gcc13 appear to include Wswitch with Wall, while
clang doesn't. For switch() statements on enum, this forces
the use of at least a default: clause, in adherance with style(9).

Reviewed By:	emaste
Sponsored by:	NetApp, Inc.
Differential Revision:	https://reviews.freebsd.org/D44092

sys/conf/kern.mk

@@ -6,7 +6,7 @@ CWARNFLAGS?=	-Wall -Wstrict-prototypes \
 		-Wmissing-prototypes -Wpointer-arith -Wcast-qual \
 		-Wundef -Wno-pointer-sign ${FORMAT_EXTENSIONS} \
 		-Wmissing-include-dirs -fdiagnostics-show-option \
-		-Wno-unknown-pragmas \
+		-Wno-unknown-pragmas -Wswitch \
 		${CWARNEXTRA}
 #
 # The following flags are next up for working on: