package/ecryptfs-utils: fix parallel build

package/ecryptfs-utils: fix parallel build

Build randomly fails since December 2017 on:

make[5]: Entering directory '/home/buildroot/autobuild/instance-2/output-1/build/ecryptfs-utils-111/src/utils'
 /bin/mkdir -p '/home/buildroot/autobuild/instance-2/output-1/target/sbin'
  /bin/bash ../../libtool   --mode=install /usr/bin/install -c mount.ecryptfs umount.ecryptfs mount.ecryptfs_private '/home/buildroot/autobuild/instance-2/output-1/target/sbin'
libtool: install: /usr/bin/install -c mount.ecryptfs /home/buildroot/autobuild/instance-2/output-1/target/sbin/mount.ecryptfs
/usr/bin/install: cannot create regular file '/home/buildroot/autobuild/instance-2/output-1/target/sbin/mount.ecryptfs': File exists
Makefile:832: recipe for target 'install-rootsbinPROGRAMS' failed
make[5]: *** [install-rootsbinPROGRAMS] Error 1

Fixes:
 - http://autobuild.buildroot.org/results/2063bd18830ef02dca0bed889c43639009bc688e

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>