libnv: Build PIC

Build libnv as position independent code so we can use it from shared
libraries.

MFC after:	4 weeks
Sponsored by:	Rubicon Communications, LLC ("Netgate")
Differential Revision:	https://reviews.freebsd.org/D29561

lib/libnv/Makefile

@@ -10,6 +10,7 @@ SHLIB_MAJOR= 0
 
 .PATH: ${SRCTOP}/sys/contrib/libnv ${SRCTOP}/sys/sys
 CFLAGS+=-I${.CURDIR}
+CFLAGS+=-fPIC
 
 SRCS=	cnvlist.c
 SRCS+=	dnvlist.c