A static-linked binary doesn't have a .dynamic section, but when
starting ujail with -r or -w will automatically search for PT_DYNAMIC in
ELF and exit with failure if it is not found.
Fixes: #970
Signed-off-by: Yuteng Zhong <zonyitoo@qq.com>
Signed-off-by: Tianling Shen <cnsztl@immortalwrt.org>