libsdl2: disable SDL's own ccache

OE has its own ccache mechanism, disable libsdl2's own ccache
searching/enabling mechanism. Otherwise, if ccache is installed
on system, it will be used by default.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>