[private] add libevent1 and sqlite3 include files for our private libraries.

This, like including ucl private headers, is useful for writing new base
system tools.  Yes, anyone using these libraries shouldn't assume ABI
compatibility.

Reviewed by:	bdrewery, bapt
Differential Revision:	https://reviews.freebsd.org/D10123

etc/mtree/BSD.usr.dist

@@ -11,6 +11,10 @@
         private
             bsdstat
             ..
+            event
+            ..
+            sqlite3
+            ..
             ucl
             ..
         ..

lib/libevent/Makefile

@@ -10,7 +10,7 @@ SHLIB_MAJOR=	1
 PRIVATELIB=
 
 SRCS=	buffer.c evbuffer.c event.c kqueue.c log.c poll.c select.c signal.c
-HDRS=	event.h
+INCS=	event.h
 
 CFLAGS+= -I${.CURDIR} \
 	-DHAVE_CLOCK_GETTIME \

lib/libsqlite3/Makefile

@@ -7,6 +7,7 @@ SHLIB_MAJOR?=	0
 LIBADD+=	pthread
 
 SRCS=	sqlite3.c
+INCS=	sqlite3.h sqlite3ext.h
 
 SQLITE=	${SRCTOP}/contrib/sqlite3
 .PATH:	${SQLITE}