Btrfs-progs: Relocate -lpthread in makefile

Btrfs-progs: Relocate -lpthread in makefile

This patch fixes the following compile error when compiled with
gcc version 4.6.1 (Ubuntu/Linaro 4.6.1-9ubuntu3):

gcc -lpthread -g -O0 -o btrfs btrfs.o btrfs_cmds.o scrub.o \
		ctree.o disk-io.o radix-tree.o extent-tree.o print-tree.o root-tree.o dir-item.o file-item.o inode-item.o inode-map.o crc32c.o rbtree.o extent-cache.o extent_io.o volumes.o utils.o btrfs-list.o btrfslabel.o  -luuid
scrub.o: In function `scrub_start':
/home/arnd/Projekte/kernel/btrfs-progs/scrub.c:1342: undefined reference to `pthread_create'
/home/arnd/Projekte/kernel/btrfs-progs/scrub.c:1360: undefined reference to `pthread_create'
/home/arnd/Projekte/kernel/btrfs-progs/scrub.c:1374: undefined reference to `pthread_join'
/home/arnd/Projekte/kernel/btrfs-progs/scrub.c:1430: undefined reference to `pthread_cancel'
/home/arnd/Projekte/kernel/btrfs-progs/scrub.c:1432: undefined reference to `pthread_join'
collect2: ld returned 1 exit status
make: *** [btrfs] Error 1

The gcc man page says: "[...] the placement of the -l option is significant." so lets include -lpthread together with the usual $(LIBS)

Signed-off-by: Arnd Hannemann <arnd@arndnet.de>
Signed-off-by: Chris Mason <chris.mason@oracle.com>