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

package/network/utils/iproute2/patches/175-reduce-dynamic-syms.patch

@@ -26,13 +26,14 @@
  
  q_atm.so: q_atm.c
  	$(QUIET_CC)$(CC) $(CFLAGS) $(CPPFLAGS) $(LDFLAGS) -shared -fpic -o q_atm.so q_atm.c -latm
-@@ -205,4 +206,15 @@ static-syms.h: $(wildcard *.c)
+@@ -205,4 +206,16 @@ static-syms.h: $(wildcard *.c)
  		sed -n '/'$$s'[^ ]* =/{s:.* \([^ ]*'$$s'[^ ]*\) .*:extern char \1[] __attribute__((weak)); if (!strcmp(sym, "\1")) return \1;:;p}' $$files ; \
  	done > $@
  
 +else
 +
 +tc: dynsyms.list
++m_xt.so: dynsyms.list
 +dynsyms.list: $(wildcard *.c)
 +	files="$(filter-out $(patsubst %.so,%.c,$(TCSO)), $^)" ; \
 +	echo "{" > $@ ; \