Can't build -current incrementally due to libnv changes

[Andrey Chernov]

Yes, 'make includes' (and 'make obj' and 'make depend') is done before
'make all' in /usr/src/lib
===> libnv (all)
cc  -O2 -pipe -march=sandybridge  -I/usr/src/lib/libnv/../../sys
-I/usr/src/lib/libnv -MD  -MF.depend.cnvlist.o -MTcnvlist.o -std=gnu99
-fstack-protector-strong -Wsystem-headers -Werror -Wall -Wno-format-y2k
-W -Wno-unused-parameter -Wstrict-prototypes -Wmissing-prototypes
-Wpointer-arith -Wreturn-type -Wcast-qual -Wwrite-strings -Wswitch
-Wshadow -Wunused-parameter -Wcast-align -Wchar-subscripts
-Winline -Wnested-externs -Wredundant-decls -Wold-style-definition
-Wno-pointer-sign -Wmissing-variable-declarations -Wthread-safety
-Wno-empty-body -Wno-string-plus-int -Wno-unused-const-variable
-Qunused-arguments  -c
/usr/src/lib/libnv/../../sys/contrib/libnv/cnvlist.c -o cnvlist.o
/usr/src/lib/libnv/../../sys/contrib/libnv/cnvlist.c:49:10: fatal error:
      'sys/cnv.h' file not found
#include <sys/cnv.h>
         ^
1 error generated.
*** Error code 1