net/dhcpcd: fix build on GCC architectures

Use C11 compiler:
arp.c: In function 'arp_found':
arp.c:145: error: #pragma GCC diagnostic not allowed inside functions
arp.c:146: error: #pragma GCC diagnostic not allowed inside functions
arp.c:148: warning: assignment from incompatible pointer type
arp.c:149: error: #pragma GCC diagnostic not allowed inside functions

Approved by: mentors (implicit approval)