erofs-utils: lib: Explicitly include <pthread.h> where used

erofs-utils: lib: Explicitly include <pthread.h> where used

compress.c and inode.c use pthread functions but do not explicitly include <pthread.h>.
This causes build failures with the Android build system,
which throws "error: implicit declaration of function pthread_*".

Signed-off-by: Satoshi Niwa <niwa@google.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20241008060819.2442945-1-niwa@google.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>