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>

arch/s390/boot/compressed/decompressor.h

@@ -2,8 +2,10 @@
 #ifndef BOOT_COMPRESSED_DECOMPRESSOR_H
 #define BOOT_COMPRESSED_DECOMPRESSOR_H
 
+#include <linux/stddef.h>
+
 #ifdef CONFIG_KERNEL_UNCOMPRESSED
-static inline void *decompress_kernel(void) {}
+static inline void *decompress_kernel(void) { return NULL; }
 #else
 void *decompress_kernel(void);
 #endif