Silence platformheaders syncqt warning about qt_egl_p.h

Drop the include for qt_egl_p.h. For Qt itself this should have no effect
since platform plugins including this header include EGL headers on their
own anyway.

Similarly, applications relying on such advanced functionality will likely
include EGL/OpenGL headers on their own - the point is anyhow to interoperate
with native, non-Qt EGL and GL code.

This avoids a lot of hassle since normally no EGL (or other winsys interface
API) bits are exposed in the public Qt APIs, and thus there are no public
headers provided to set up EGL headers in the same way Qt does internally.

Change-Id: Icdbc28811b753799abc06085bc8dff7f09bdbff9
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/platformheaders/nativecontexts/qeglnativecontext.h

@@ -41,7 +41,8 @@
 #define QEGLNATIVECONTEXT_H
 
 #include <QtCore/QMetaType>
-#include <QtEglSupport/private/qt_egl_p.h>
+
+// Leave including egl.h with the appropriate defines to the client.
 
 QT_BEGIN_NAMESPACE
 

src/platformheaders/nativecontexts/qeglnativecontext.qdoc

@@ -36,6 +36,12 @@
   that an application using it is only guaranteed to work with the Qt version it was
   developed against.
 
+  \note Due to being public while relying on otherwise hidden EGL types, this header
+  itself does not include \c{EGL/egl.h}. It is the application's responsibility to
+  include egl.h with any appropriate defines (for example, \c{MESA_EGL_NO_X11_HEADERS}
+  or other vendor-specific defines controlling the typedefs for EGL's native resources)
+  before this header.
+
   \sa QOpenGLContext::setNativeHandle(), QOpenGLContext::nativeHandle()
  */