iproute2: m_xt.so depends on dynsyms.list

iproute2: m_xt.so depends on dynsyms.list

When doing parallel build on a fast machine with bottleneck in i/o,
m_xt.so may start linking faster than dynsyms.list gets populated,
resulting in error:

ld:dynsyms.list:0: syntax error in dynamic list

Fix this by adding dynsyms.list as make dependency to m_xt.so
Described also here:
https://bugs.openwrt.org/index.php?do=details&task_id=3353

Change from v1:
- add dynsysms.list dependancy only when shared libs are enabled

Signed-off-by: Roman Yeryomin <roman@advem.lv>
Fixes: FS#3353