bpftool: Skeleton should depend on libbpf

Add the dependency to libbpf, to fix build errors like:

  In file included from skeleton/profiler.bpf.c:5:
  .../bpf_helpers.h:5:10: fatal error: 'bpf_helper_defs.h' file not found
  #include "bpf_helper_defs.h"
           ^~~~~~~~~~~~~~~~~~~
  1 error generated.
  make: *** [skeleton/profiler.bpf.o] Error 1
  make: *** Waiting for unfinished jobs....

Fixes: 47c09d6a ("bpftool: Introduce "prog profile" command")
Suggested-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200312182332.3953408-3-songliubraving@fb.com

tools/bpf/bpftool/Makefile

@@ -126,11 +126,12 @@ $(OUTPUT)_prog.o: prog.c
 $(OUTPUT)_bpftool: $(_OBJS) $(LIBBPF)
 	$(QUIET_LINK)$(CC) $(CFLAGS) $(LDFLAGS) -o $@ $(_OBJS) $(LIBS)
 
-skeleton/profiler.bpf.o: skeleton/profiler.bpf.c
+skeleton/profiler.bpf.o: skeleton/profiler.bpf.c $(LIBBPF)
 	$(QUIET_CLANG)$(CLANG) \
 		-I$(srctree)/tools/include/uapi/ \
 		-I$(srctree)/tools/testing/selftests/bpf/include/uapi \
-		-I$(srctree)/tools/lib -g -O2 -target bpf -c $< -o $@
+		-I$(LIBBPF_PATH) -I$(srctree)/tools/lib \
+		-g -O2 -target bpf -c $< -o $@
 
 profiler.skel.h: $(OUTPUT)_bpftool skeleton/profiler.bpf.o
 	$(QUIET_GEN)$(OUTPUT)./_bpftool gen skeleton skeleton/profiler.bpf.o > $@