tools/mm: fix undefined reference to pthread_once

Commit 97d5f2e ("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/[email protected]
Fixes: 97d5f2e ("tools api fs: More thread safety for global filesystem variables")
Signed-off-by: Xie XiuQi <[email protected]>
Acked-by: Ian Rogers <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

tools/mm/Makefile

@@ -8,8 +8,8 @@ TARGETS=page-types slabinfo page_owner_sort
 LIB_DIR = ../lib/api
 LIBS = $(LIB_DIR)/libapi.a
 
-CFLAGS += -Wall -Wextra -I../lib/
-LDFLAGS += $(LIBS)
+CFLAGS += -Wall -Wextra -I../lib/ -pthread
+LDFLAGS += $(LIBS) -pthread
 
 all: $(TARGETS)