coova-chilli: Fix compile problem with _init()

coova-chilli: Fix compile problem with _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.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>