pixman: add patch to fix build issue with musl

pixman fails to build with musl because <float.h> is included in
assembler files, which doesn't work with the <float.h> provided by
musl. This commit fixes that by patching pixman (patch submitted
upstream).

Reported-by: Eial Czerwacki <eial@scalemp.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>