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
i386 jessie debian
debian_jessie_i386_deb

00:32:22

passed #10557
x86_64 jessie debian
debian_jessie_x86_64_deb

00:38:40

passed #10558
x86_64 stretch debian
debian_stretch_x86_64_deb

00:33:36

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

00:23:52

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

00:22:11

passed #10565
stretch arm raspbian
raspbian_stretch_armv7l_deb

00:54:51

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

00:40:57

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

00:38:52

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

00:49:45

passed #10564
i386 windows
windows_i386_innosetup

01:29:31