ti-gfx: use only -I${includedir} as pkg-config Cflags

Using only -I${includedir} in common practice.

Settings only -I${includedir}/GLES2 breaks Qt5's CMake files,
because at configure time CMake only searchs the paths reported
by pkg-config, and not even /usr/include is used as default.

Even though pkg-config strips out standard include path, that's not
the case with pkgconf (which we are using) in cross-compilation,
which correctly reports the /usr/include dir prefixed with the
sysroot.

Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Kümmel <syntheticpp@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>