tools vm: Fix build due to removal of tools/lib/api/fs/debugfs.h

There were some changes in how this debugfs mounting helper is
implemented/exported and we forgot to check if there were other users
besides perf, fix it.

Need to do a make -C tools/ everytime we do changes to
tools/{lib,include} and other places where we're moving things from
tools/perf/ to be used by other tools/ living code.

Fixed:

  $ make -C tools/vm
  make: Entering directory '/home/git/linux/tools/vm'
  make -C ../lib/api
  make[1]: Entering directory '/home/git/linux/tools/lib/api'
    CC       fd/array.o
    LD       fd/libapi-in.o
    CC       fs/fs.o
    CC       fs/tracing_path.o
    LD       fs/libapi-in.o
    CC       cpu.o
    LD       libapi-in.o
    AR       libapi.a
  make[1]: Leaving directory '/home/git/linux/tools/lib/api'
  gcc -Wall -Wextra -I../lib/ -o page-types page-types.c ../lib/api/libapi.a
  make: Leaving directory '/home/git/linux/tools/vm'
  $

Reported-by: Vinson Lee <vlee@twopensource.com>
Tested-by: Vinson Lee <vlee@twopensource.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Raphael Beamonte <raphael.beamonte@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: David Ahern <dsahern@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Fixes: 60a1133a ("tools lib api fs: Remove debugfs, tracefs and findfs objects")
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>