shared/platform: avoid build warning when building without gl-renderer

When building without GL renderer the compiler prints the following
warning:
  ../clients/subsurfaces.c: In function ‘egl_state_create’:
  ../clients/subsurfaces.c:225:35: warning: passing argument 1 of
  ‘weston_platform_get_egl_display’ makes pointer from integer without a
  cast [-Wint-conversion]
    225 |   weston_platform_get_egl_display(EGL_PLATFORM_WAYLAND_KHR,
        |                                   ^~~~~~~~~~~~~~~~~~~~~~~~
        |                                   |
        |                                   int
...

Define the fallback implementation of weston_platform_get_egl_display
to take an integer which is the underlaying datatype of EGLenum.

Signed-off-by: Stefan Agner <stefan@agner.ch>

shared/platform.h

@@ -138,7 +138,7 @@ weston_platform_destroy_egl_surface(EGLDisplay display,
 #else /* ENABLE_EGL */
 
 static inline void *
-weston_platform_get_egl_display(void *platform, void *native_display,
+weston_platform_get_egl_display(int platform, void *native_display,
 				const int *attrib_list)
 {
 	return NULL;