external,Platform: suppress GL deprecation warnigs on iOS as well.

Got done in 7ced4740fd and
f3fc17e2b0 for CGL and OpenAL, but ever
since Travis decided we're not worthy of using their services, I didn't
have a chance to check the warning in the iOS build. Now I have.

src/Magnum/Platform/WindowlessIosApplication.mm

@@ -31,6 +31,7 @@
 #include "Magnum/GL/Version.h"
 #include "Magnum/Platform/GLContext.h"
 
+#define GLES_SILENCE_DEPRECATION /* YES I KNOW, APPLE! FFS */
 #import "EAGL.h"
 
 #if __has_feature(objc_arc)

src/MagnumExternal/OpenGL/GLES2/flextGLPlatformIOS.cpp

@@ -162,6 +162,7 @@
 #undef glGenVertexArraysOES
 #undef glIsVertexArrayOES
 
+#define GLES_SILENCE_DEPRECATION /* YES I KNOW, APPLE! FFS */
 #include <OpenGLES/ES2/glext.h>
 
 void flextGLInit(Magnum::GL::Context&) {

src/MagnumExternal/OpenGL/GLES3/flextGLPlatformIOS.cpp

@@ -160,6 +160,7 @@
 #undef glTexStorage3DMultisampleOES
 #undef glFramebufferTextureMultiviewOVR
 
+#define GLES_SILENCE_DEPRECATION /* YES I KNOW, APPLE! FFS */
 #include <OpenGLES/ES3/glext.h>
 
 void flextGLInit(Magnum::GL::Context&) {