mqueue: Export kern_kmq_* symbols from kernel module

linux.ko supports POSIX message queues using these symbols, so they need
to be exported rather than relying on the kernel linker's misfeature of
linking against debug or local symbols (which will soon be disabled by
default).

Reported by:	mav
Reviewed by:	zlei
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D53907

sys/modules/mqueue/Makefile

@@ -5,4 +5,11 @@ SRCS=	uipc_mqueue.c \
 	vnode_if.h \
 	opt_posix.h opt_capsicum.h
 
+EXPORT_SYMS=	\
+	kern_kmq_notify \
+	kern_kmq_open \
+	kern_kmq_setattr \
+	kern_kmq_timedreceive \
+	kern_kmq_timedsend
+
 .include <bsd.kmod.mk>