Otherwise, on some environments, GNU/Make may try to expand
implicit rules right in the top Makefile.

Fixes:
[criu]$ make criu/parasite-syscall.o
...
make -r -R -f /criu/scripts/nmk/scripts/main.mk makefile=Makefile obj=criu criu/parasite-syscall.c
make[1]: Entering directory `/criu'
make[1]: Nothing to be done for `criu/parasite-syscall.c'.
make[1]: Leaving directory `/criu'
gcc   -O2 -g -Wall -Wformat-security -Werror  -D_FILE_OFFSET_BITS=64 -D_GNU_SOURCE   -c -o criu/parasite-syscall.o criu/parasite-syscall.c
criu/parasite-syscall.c:8:22: fatal error: protobuf.h: No such file or directory
 #include "protobuf.h"
                      ^
compilation terminated.

Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>