mesa3d: fix khrplatform.h installation.

Mesa3D only installs khrplatform.h if EGL support is enabled, whereas
this header file is needed by the headers of OpenGL|ES and OpenVG.

The supplied patch forces Mesa3D to install this header unconditionally.

Fixes http://autobuild.buildroot.net/results/e07/e079219d70830a123977c3ee09d3c11b863d0691



Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>