[SDL] compiler warnings

[M. Gerhardy]

Hey guys - as the release is currently in preparation - here are some
warnings that you might wanna fix.

src/video/SDL_egl.c:201:22: warning: assigning to 'char *' from 'const char
[15]' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
path = "libGLESv2.so.2";
^ ~~~~~~~~~~~~~~~~
src/video/SDL_egl.c:204:22: warning: assigning to 'char *' from 'const char
[18]' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
path = "libGLESv1_CM.so.1";
^ ~~~~~~~~~~~~~~~~~~~
src/video/SDL_egl.c:207:26: warning: assigning to 'char *' from 'const char
[16]' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
path = "libGLES_CM.so.1";
^ ~~~~~~~~~~~~~~~~~
src/video/SDL_egl.c:214:18: warning: assigning to 'char *' from 'const char
[11]' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
path = "libGL.so.1";
^ ~~~~~~~~~~~~
src/video/SDL_egl.c:236:18: warning: assigning to 'char *' from 'const char
[12]' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
path = "libEGL.so.1";

src/core/linux/SDL_dbus.c:30:33: warning: missing field 'bus_get_private'
initializer [-Wmissing-field-initializers]
static SDL_DBusContext dbus = {0};
^
1 warning generated.
[ 22%] Building C object CMakeFiles/sdl2.dir/src/core/linux/SDL_evdev.c.o
[ 22%] Building C object CMakeFiles/sdl2.dir/src/core/linux/SDL_ibus.c.o
src/core/linux/SDL_ibus.c:45:38: warning: missing field 'y' initializer
[-Wmissing-field-initializers]
static SDL_Rect ibus_cursor_rect = {0};
^
src/core/linux/SDL_ibus.c:344:42: warning: missing field 'message_function'
initializer [-Wmissing-field-initializers]
DBusObjectPathVTable ibus_vtable = {0};
^
2 warnings generated.