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 bbe847a

[coveralls]

Coverage decreased (-6.6%) to 72.015% when pulling c7f84aa on gsnw:buildwarn/fping-c_2078-12 into 5d3eee1 on schweikert:develop.

[gsnw]

Obsolet fix by another commit