Superfluous variable response_addr_len in function decode_icmp_ipv4

[gsnw]

Fixes the following build warning

fping.c: In function ‘decode_icmp_ipv4’:
fping.c:2078:12: warning: unused parameter ‘response_addr_len’ [-Wunused-parameter]
     size_t response_addr_len,
     ~~~~~~~^~~~~~~~~~~~~~~~~

The variable response_addr_len in function decode_icmp_ipv4 has become redundant with the commit https://github.com/schweikert/fping/commit/bbe847a5d9ae20b5e27689cd485e62649fb32491

[coveralls]

Coverage decreased (-6.6%) to 72.015% when pulling c7f84aa458bda22d4f42006de4650a40494531d4 on gsnw:buildwarn/fping-c_2078-12 into 5d3eee1105e2559419a3820bb71d66fc44a2de55 on schweikert:develop.

[gsnw]

Obsolet fix by another commit