btrfs-tools: fix symlink creation multiple times

btrfs-tools: fix symlink creation multiple times

The rule to create symlink in Makefile caused parallel issue:
$ make -j 40 DESTDIR=/image install BUILD_VERBOSE=1
...
  1     [LN]     libbtrfs.so.0
  2     [LN]     libbtrfs.so
  3 ln -s -f libbtrfs.so.0.1 libbtrfs.so.0
  4 ln -s -f libbtrfs.so.0.1 libbtrfs.so.0
  5 ln -s -f libbtrfs.so.0.1 libbtrfs.so
  6 ln -s -f libbtrfs.so.0.1 libbtrfs.so
...

It failed occasionally:
...
|symlinkat: couldn't stat 'git/libbtrfs.so' even though symlink
creation succeeded (No such file or directory).
|ln: failed to create symbolic link 'libbtrfs.so': No such file or directory
...

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>