libxt_coova: Use constructor instead of _init()

The _init() function collides with a _init() function in crti.o.
This results in the following error in OpenWrt:
arc-openwrt-linux-gnu/bin/ld: libxt_coova.o: in function `_init':
/lib/gcc/arc-openwrt-linux-gnu/11.3.0/crti.o:(.init+0x4): first defined here
collect2: error: ld returned 1 exit status
make[7]: *** [Makefile:22: libxt_coova.so] Error 1

Fix this by suing a constructor attribute instead. This function will
still be called after the shared library was loaded.

Fixes: #553
Signed-off-by: Hauke Mehrtens <[email protected]>

src/linux/libxt_coova.c

@@ -126,7 +126,7 @@ static struct xtables_match coova_mt6_reg = {
 	.extra_opts    = coova_opts,
 };
 
-void _init(void)
+void __attribute__ ((constructor)) xtables_init(void)
 {
 	xtables_register_match(&coova_mt_reg);
 	xtables_register_match(&coova_mt6_reg);