fix typo in makefile flag

10 jobs for port-knob-squashed in 369 minutes and 52 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #10556
debian jessie i386
debian_jessie_i386_deb

00:32:22

passed #10557
debian jessie x86_64
debian_jessie_x86_64_deb

00:38:40

passed #10558
debian stretch x86_64
debian_stretch_x86_64_deb

00:33:36

passed #10562
osx v10.11 dmg
osx_10.11_x86_64_dmg

00:23:52

passed #10563
osx v10.8 dmg
osx_10.8_x86_64_dmg

00:22:11

passed #10565
raspbian arm stretch
raspbian_stretch_armv7l_deb

00:54:51

passed #10559
ubuntu v14.04 i386
ubuntu_14.04_i386_deb

00:40:57

passed #10560
ubuntu v14.04 x86_64
ubuntu_14.04_x86_64_deb

00:38:52

passed #10561
ubuntu v16.04 x86_64
ubuntu_16.04_x86_64_deb

00:49:45

passed #10564
windows i386
windows_i386_innosetup

01:29:31