package/qemu: fix typo on STAGING_DIR

Since this commit from 2010:
'87b81bb56f toolchain: move sysroot to host dir'
BR2_STAGING_DIR does not exist anymore.
So fix the typo added in 2014 with commit:
8956c336eb package/qemu: add SDL frontends

Cc: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/qemu/qemu.mk

@@ -72,7 +72,7 @@ endif
 ifeq ($(BR2_PACKAGE_QEMU_SDL),y)
 QEMU_OPTS += --enable-sdl
 QEMU_DEPENDENCIES += sdl2
-QEMU_VARS += SDL2_CONFIG=$(BR2_STAGING_DIR)/usr/bin/sdl2-config
+QEMU_VARS += SDL2_CONFIG=$(STAGING_DIR)/usr/bin/sdl2-config
 else
 QEMU_OPTS += --disable-sdl
 endif