s390/decompressor: fix build warning

Fixes the following warning with CONFIG_KERNEL_UNCOMPRESSED=y

arch/s390/boot/compressed/decompressor.h:6:46: warning: non-void function
does not return a value [-Wreturn-type]
static inline void *decompress_kernel(void) {}
                                             ^

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>