sdl2: fix sdl2.pc include paths

SDL2 dumps all cflags to the sdl2.pc, which might then contain paths
to the recipe sysroot include dir. This happens at least with
raspberrypi.

Change-Id: I77139bd31956039d4b8db947161f032fb8fbdaf7
Reviewed-by: Mikko Gronoff <mikko.gronoff@qt.io>

meta-boot2qt-distro/recipes-graphics/libsdl2/libsdl2_%.bbappend

@@ -28,3 +28,7 @@
 ############################################################################
 
 PACKAGECONFIG_GL = "${@bb.utils.contains('DISTRO_FEATURES', 'opengl', 'gles2', '', d)}"
+
+do_install_append() {
+    sed -i -e "s|${STAGING_INCDIR}|$\{includedir}|g" ${D}${libdir}/pkgconfig/sdl2.pc
+}