tools/mm: fix undefined reference to pthread_once

tools/mm: fix undefined reference to pthread_once

Commit 97d5f2e9ee12 ("tools api fs: More thread safety for global
filesystem variables") introduces pthread_once, so the libpthread
should be added at link time, or we'll meet the following compile
error when 'make -C tools/mm':

  gcc -Wall -Wextra -I../lib/ -o page-types page-types.c ../lib/api/libapi.a
  ~/linux/tools/lib/api/fs/fs.c:146: undefined reference to `pthread_once'
  ~/linux/tools/lib/api/fs/fs.c:147: undefined reference to `pthread_once'
  ~/linux/tools/lib/api/fs/fs.c:148: undefined reference to `pthread_once'
  ~/linux/tools/lib/api/fs/fs.c:149: undefined reference to `pthread_once'
  ~/linux/tools/lib/api/fs/fs.c:150: undefined reference to `pthread_once'
  /usr/bin/ld: ../lib/api/libapi.a(libapi-in.o):~/linux/tools/lib/api/fs/fs.c:151:
  more undefined references to `pthread_once' follow
  collect2: error: ld returned 1 exit status
  make: *** [Makefile:22: page-types] Error 1

Link: https://lkml.kernel.org/r/20230831034205.2376653-1-xiexiuqi@huaweicloud.com
Fixes: 97d5f2e9ee12 ("tools api fs: More thread safety for global filesystem variables")
Signed-off-by: Xie XiuQi <xiexiuqi@huawei.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>