nginx: fix patch using pkg-config

The regular expression that drops the -I flag from the output of
pkg-config --cflags-only-I doesn't work properly as it is too naive:
it breaks a path if it contains -I:

$ echo "-I/usr/Some-Install-Location/include" | sed 's/-I//g'
/usr/Somenstall-Location/include
$ echo "-I/usr/Some-Install-Location/include" | sed -re 's/(^|\s)-I\s*(\S+)/\1\2/g'
/usr/Some-Install-Location/include

Signed-off-by: Ignacy Gawędzki <ignacy.gawedzki@green-communications.fr>
[Thomas: improved commit log.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>