qextserialport: install QExtSerialPort header with qmake

Instead of manually copying the QExtSerialPort header introduced in
patch 0002, update the patch to include this header in the qmake
project file so it gets properly installed.

While we're at it, convert to a git-formatted patch.

Fixes:
http://autobuild.buildroot.net/results/818b564a6e23bcecf4afc7e53811227b08e5f4eb
http://autobuild.buildroot.net/results/0b6c2d7c63329f5e790f02ccd29f14a317eadc71
http://autobuild.buildroot.net/results/ac552b63c6fb099a136066cda421f68e639808fa



Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>