platform: explicitly cast the return value of weston_platform_get_egl_proc_address

This allows the header to be consumed by C++ compilers, because C++ does
away with C's implicit cast from (void*).
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>

shared/platform.h

@@ -75,7 +75,8 @@ weston_platform_get_egl_display(EGLenum platform, void *native_display,
 	static PFNEGLGETPLATFORMDISPLAYEXTPROC get_platform_display = NULL;
 
 	if (!get_platform_display) {
-		get_platform_display = weston_platform_get_egl_proc_address(
+		get_platform_display = (PFNEGLGETPLATFORMDISPLAYEXTPROC)
+            weston_platform_get_egl_proc_address(
                 "eglGetPlatformDisplayEXT");
 	}
 
@@ -95,7 +96,8 @@ weston_platform_create_egl_surface(EGLDisplay dpy, EGLConfig config,
 		create_platform_window = NULL;
 
 	if (!create_platform_window) {
-		create_platform_window = weston_platform_get_egl_proc_address(
+		create_platform_window = (PFNEGLCREATEPLATFORMWINDOWSURFACEEXTPROC)
+            weston_platform_get_egl_proc_address(
                 "eglCreatePlatformWindowSurfaceEXT");
 	}