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
(cherry-picked from commit edd53df168)

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

@@ -1,6 +1,6 @@
 --- a/tc/Makefile
 +++ b/tc/Makefile
-@@ -107,7 +107,7 @@ LDLIBS += -L. -lm
+@@ -108,7 +108,7 @@ LDLIBS += -L. -lm
  
  ifeq ($(SHARED_LIBS),y)
  LDLIBS += -ldl
@@ -9,7 +9,7 @@
  endif
  
  TCLIB := tc_core.o
-@@ -137,7 +137,7 @@ MODDESTDIR := $(DESTDIR)$(LIBDIR)/tc
+@@ -138,7 +138,7 @@ MODDESTDIR := $(DESTDIR)$(LIBDIR)/tc
  all: tc $(TCSO)
  
  tc: $(TCOBJ) $(LIBNETLINK) libtc.a
@@ -18,7 +18,7 @@
  
  libtc.a: $(TCLIB)
  	$(QUIET_AR)$(AR) rcs $@ $^
-@@ -159,6 +159,7 @@ install: all
+@@ -160,6 +160,7 @@ install: all
  clean:
  	rm -f $(TCOBJ) $(TCLIB) libtc.a tc *.so emp_ematch.yacc.h; \
  	rm -f emp_ematch.yacc.*
@@ -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
-@@ -198,4 +199,15 @@ static-syms.h: $(wildcard *.c)
+@@ -199,4 +200,16 @@ 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 "{" > $@ ; \